So...I have a Mark IVa, and its had a lot of reliability issues. Within the first year of owning it, I had to send it back to Mesa twice to eventually get all of the Orange drop caps in the amp replaced. I have owned the amp less than 2 years total at this point, and currently it is having an issue with randomly switching channels in rapid succession. Because of this, I can't trust this amp on stage. However, it is my favorite sounding amp that I have owned... So I started looking for something newer that would be more reliable to replace the Mark IV as a live amp. Bought a KSR Orthos, and decided that wasn't the amp for me. Saw this Mark IVb in the classifieds on RT, but it was pending sale, but let the seller know that if that didn't work out to let me know. Next day, I sent payment for the amp...and it just arrived last night. First thing I did was pull the EL34s out LOL...then dialed it in to the same settings that I typically use on my IVa. Noticed 3 things. 1...the b has more gain. For sure. Noticeably more. On my a, I usually have the lead gain, lead drive, and treble set at about 8 to get the level of saturation that I like. I had to back all of those down to between 6.5 and 7 on the b to match the gain level of the a...cool! I love gain haha. Next, the amp is VERY SLIGHTLY less bright compared to the a. Easy enough to up the presence by a number and up the high slider...and then its basically identical. I mean, this is a hair splitting difference...barely noticeable, and I would probably think that anyone with no Mark experience would not notice. 3...the b is not quite as massively room filling as the a. Again, splitting hairs. Its so close that most people would not notice, and I am betting that when I get to swapping preamp tubes around, this will change. Otherwise, these amps sound identical. I can't tell a difference between the clean channels. Havent used R2 on it yet, but I will next time I record something, since I use R2 as a bass amp for recording lol. Amp was packed just the way I would pack it. This thing is INSANELY clean. It looks damn near brand new...barely even any dust on it. If I were to buy this from Mesa, I would have thought it was NOS. Its THAT clean. Amazing. Anyway...I love it, and having 2 Mark IVs is definitely the right answer for me. So when the issues with my a get to unmanageable levels, I can send it in and still have the b working. Plan right now is to keep the b as a band practice and live amp and use the a as my at home jamming and recording amp. Will probably be looking for a Mark III next year. Nothing like a Mark!