Sorry for the long post.... :roll:
The Mesa branded Celestion speakers should not take that long to break in. I thought the 1x12 open back cab sounded amazing right out of the box with the MC90 (stock speaker). There is a break in period with just about any speaker, some takes much longer than others. Amps also have a break in period but it is not the same thing as a electro-mechanical relationship. It is related to the power supply capacitors. From the start they will be at their higher value within the tolerance ratings. After a few thermal cycles of running the amp and turning it off the large electrolytic capacitors will get closer to there nominal values. Capacitors will drift over time but will take many decades to require replacement. Speakers also change over the life span but the change is usually gradual and may never be noticed. Tubes on the other hand always start off as bright, bold and whatever its characteristic is for that tube type. Those too have a burn in period where the tone will shift just a bit and then the change will be gradual over the span of the tube life. This is more common with power tubes than preamp tubes but those to shift in their performance but at a slower rate. My Mark V does not sound the same as it did in 2012, it used to be extremely bright and sometimes brittle, tubes did not seem to matter much, have many speaker cabs to use. Input voltage from the line can be a major factor. My home is a bit on the high side. so a power conditioner on the outlet I have my gear plugged in will keep things where it should be. The Mark V is very sensitive to higher input voltage, 110V is ideal, unfortunately my line runs at 120 -125 VAC. A regulated supply really helped me with the Mark V as well as my other gear as it also eliminates noises from the high current equipment like the central heating / cooling system.
For starters, use the 45W mode on CH3 as that has an amazing tone since you are cutting out the overhead of the extra Class A/B tubes. Also note, 45W power mode gives you the option to select tube rectification for CH1 and CH2, this will reduce the output by -2dB or -3dB when compared to the silicon diode mode but the additional sag due to the inefficient tube rectifier will get you closer to a more vintage tone. Even the Variac power mode is nice and spongy. If you want it tight, chose silicon diode for CH1 and CH2.
As for CH3, set all of the controls to around 10AM (gain, channel volume, presence, treble, midrange and bass). Bright switch on and on the back use pentode. I can get a very aggressive tone with this setting on Mark IV and Extreme voice. Set the 5BEQ on and use the slider setting in a V pattern. Raise the 750Hz slider up just enough (just below the line between the bottom and center). This works for me but mine is a 2012 model and can be very bright to start with. Note that you do not need to push the gain all the way on CH3 to get an awesome distortion that is detailed for lead and aggressive for chords. The higher the gain setting the muddier the tone. The bright switch on CH3 does two things, it doubles the gain factor on one tube stage and increases the treble, midrange and bass response. The word "bright" is a bit misleading. I have read many reviews on the Mark V, including John Petrucci comments. Trick is to raise the bass control just to where it gets muddy and then drop it from that point. It is okay do dial it completely out if needed as well as presence and treble. They are just tone control adjustments so all the way out or full on is up to you. If you try the common method to set all three tone controls to 50% for evaluating the tone stack, this will not work too well. The tone controls are a bit different than your typical tube amp. You can do this on CH1 as that is more of a Fender circuit but for CH2 and CH3 they are quite different.
First time I ran though the Mark V, I was exploring everything, all the switches, power modes, settings, FX loop active or in bypass, FX loop with effects... etc. The FX loop output of the 90W V is a bit stronger than the other Mark amps as well as other Mesa amps (that I know of). It is actually a line level loop vs instrument level loop. Some effect pedals may have an overloaded input buffer (tone will sound clipped on a clean channel or extremely compressed on the higher gain channels.) When shopping for effects, look for line level compliant pedals (max input 10dB down to -2dB, if it states -10dB or lower it is instrument level). If you do have effect pedals, and you find there are tone issues with them in the loop, you will need a 2 channel line level shifter. Ebtech makes one, https://www.sweetwater.com/store/detail/LLS2 I am not trying to sell the thing, but I do need to get one as I do have some pedals that are not compliant with line level. It should be easy to use, connect the +4dB jacks to the FX loop send and return of the Mark V and the pedal chain to the -10dB jacks. The device uses step up/down transformers which does not require any external power. I am uncertain if a booster pedal would be required at the end of the effects chain to compensate for dry (bypass) or wet (fx on) . I should just buy it and find out for myself. I basically sold most of my gear after I got the Mark V and bought some new pedals that were line level compliant. (Strymon is one, there are others that are line level and will also auto shift to instrument level so they work well in front of amp or in FX loop). There is a lot of things to explore with the Mark V. So far the Mark V has the best overall performance on the Clean channel in fat voice when using piezo bridge pickups. I have tried my Martin (MIM) 12 string as well as the Takamine 6 string acoustic, okay but feedback would be a huge issue as I do not have the sound hole plugged. Best is a piezo bridge on an electric guitar. I have a Kiesel DC600 with the piezo Floyd Rose ( Ghost system licensed by Graph Tech, I think the bridge itself is a GOTOH bridge, hard to say as it is not identical to the OFR bridges I have on other guitars but close enough). Piezo on a floating bridge is okay but any movement of the bridge gest transferred in to the signal but once it is fixed in position or not bumped too much it sound better than the acoustic guitars. A better design would be on a Tune-o-matic bridge than a Floyd Rose. Still it is fun to use. Some guitars with active tone controls can be used on Tweed voice with a low gain setting on CH1 and will sound acoustical in character but not like the piezo pickup. The Mark V has many features and practical uses that makes it a great tool or instrument to use. Piezo bridges do not work very well with the higher gain channels. Stick with magnetics (active or passive) for that purpose.
Trick with the Mark I mode on CH2, dial the bass completely out, reduce midrange and boost treble. This is more important when using the thick switch since the Mark I voice is very dark in its tone. Try the settings in the manual "Gain and More Gain" (on page 45 of the downloaded manual) but I would recommend reduction in the gain control on CH3 if you get too much feedback. CH2 setup is great (actually the John Petrucci setting) CH1 can be bright so expect to adjust treble and presence. Some of the controls may need to be adjusted a bit to taste.
