I believe the bottom end attribute of the Roadster is due to the late tone stack. For the two high gain channels (3 and 4) the signal path passes though 4 gain stages then gets condensed though a cathode follower to drive the tone stack. The bass gets plenty of drive since it has the most power in the signal vs the higher frequencies. Bass does not seem to be required. Clean channel only hits one gain stage before the tone stack and then gets bumped up though two gain stages (this is closer to a Mark signal path since the tone stack sits between V1A and V1B for all channels). Note that the clean channels never pass though the cathode follower circuit of V3. This also adds to the volume change between modern, vintage and raw settings. That does not happen with Clean, fat or tweed/brit voices on CH1 and CH2 . With the stock tube in V1 and V2 the CH4 is so dark it becomes laden with bottom end. Just a simple change in V1 with a tube that has a higher frequency gain like a Tung Sol prevents the bass being too dominant and the bass tone control makes sense to use it on the two gain stages.
When comparing to a Mark series amp, the signal on the high gain channels (Mark V only had one which is CH2 and the JP-2C has two CH2 and CH3) burden here is the full midrange frequencies. There is plenty of bottom and top end as well. The 5BEQ is basically there to tune out the midrange frequencies. Without the EQ in use, it is okay but you will find other amps that were not designed be tailored with a 5BEQ sound great as there is filtering to tailor the driven signal before pushing into the power section. Mesa could have designed a tuned filter for the signal chain if the EQ is not used but they did not do such as it is more of the raw signal before the EQ and in some cases may sound muddy. Similar to the Modern voice of the Roadster or Roadking being too bass dominant, if the signal is flooded with a huge hump in the midrange the signal also suffers from detail which is a Mark Trait. The JP-2C has a huge bottom end presence and plenty of punch in the midrange and the high frequencies are more tapered off which results in a pleasant grind signature (sinister, evil, pronounced, authoritative) not a bad thing for metal style of playing. The gain structure is not as severe is you would find in the Mark V (using the Mark IV voice) as the gain character really pump up the upper midrange and high frequencies and tends to cancel out the ideal tone when the gain control goes up. Depending on the year the amp was made, it may have that ice pick tone that is not very pleasing, also sounds lacking or thin with the grind signature. Grind signature being the character you get with a power chord and does it rock your world or not... The big thing with the V lately is the V4 mod (change from 12AX7 to a 12AT7) which helps to reduce the upper frequency gain dominance as the positive feedback used on the secondary cascaded gain stage is reduced thus restoring the true Mark Tone. Gets very close to the JP-2C character. The IIC+ voice on the V is a bit lower gain that what is found on the JP-2C, however the full range of characteristics of the V can easily be dialed in with the JP-2C. Instead of having a labeled toggle switch indicating a so called "amp character" you have two pulls on each gain channel (presence and gain) as well as a grid slammer function called shred. There is actually more than what meets the eye with the JP-2C. Out of box experience, the JP-2C wins. The catch...If you have not discovered it yet.... the JP-2C is biased only to operate on one tube type, mainly 6L6GC tubes. It does not have an EL34 bias switch and cannot be used to run EL34s. The Mark V can be used with EL34 as well as the Roadster, The Roadking already has the EL34 with its progressive linkage and other features. Figures I would point that out in case you had plans to run EL34 vs the 6L6 tubes.