Scooping the mids is not bad as such if you like that sound (which I do as well, I often run my Tremoverb with the mids *off*!) but it can cause other problems with mix levels and interaction with some effects, because it's the mids that really control the perceived volume, being where the human ear is most sensitive.
One of the reasons us scoop-mid guys need a lot of power and channel switching is because of this - the single-channel guys who use a lot of mids and go clean to dirty from the guitar volume can't understand why anyone would need more than 20 watts
, but if you like a really heavy distortion sound with less mids you need to be able to set the volume much higher than for the clean sound or it will actually seem quieter, and you need a lot of power behind it to get the headroom and frequency range for band volume - 100W is not too much, even if you don't apparently play that loud. The small-amp guys don't understand why anyone would need that much power and think that we must all be playing stadiums at nuclear-bomb volume... (which is what a 100W amp will do if you dime the mids and turn it right up, of course).
The wah is basically a sweepable band-pass filter, which cuts out everything except a narrow range of mid frequencies. Most wahs have a small amount of boost built-in anyway since this tends to cut the apparent volume even with a clean midrangy sound, but it then ends up not being enough if you scoop the mids at the amp, because the two things cancel out. It's only when you get the wah fully down and the frequency center moves up above the mid scoop that you suddenly get volume. So to fix this you need to add more boost in the wah.
If you don't want to change the wah, three channels will help exactly as said because you can set a heavy rhythm sound with no mids and a lead sound with lots, then the wah won't kill the volume for leads. A 3-channel Rectifier will give you that. I think you'll find it's a much better amp than the Single in several ways - not that the Single is bad, it just has limitations that the Dual doesn't. You'll also get an inherently bigger, deeper sound with the Dual and you may not actually need to cut the mids quite as much.
For what it's worth, when I'm playing live with the Tremoverb I normally only use it as a clean and crunch amp with the two onboard channels - I use a pedal for leads, which gives a lot more midrange and a tighter sound... you'll find a lot of the 2-channel Dual Rectifier guys do this. Recording is another thing entirely, you can set the amp in different ways for the different sounds.
