I’ve owned the Mezzo XD’s longer than anyone. I’ve tried numerous amps on them. Way too many to mention. I did try the Air Tight 300b’s and they were noisy and thin sounding.
The best I’ve tried were the Triode TRX-M300’s (300b) and VAC Signature 200iQ with vintage GoldLion KT88’s. There are still many I would love to try. If you value black backgrounds, stick with a balanced, push pull tube amp. For the record, I have not found a solid state amp I like on the Duo Mezzo’s. The Pass XA25 is as close as I’ve found so far. I’ve tried three OTL’s and they were good, but their lack of a transformer made them sound a little thin to my ears.
Downsides? They take time to get right, but once you do, you will be rewarded in spades. You must have good clean power, quiet electronics and be willing to spend time on setup and their EQ for the bass.
If you have something like 500 watt Levinson mono amps, Nordost cables, etc. and you’re not willing to change, then they aren’t for you. If you don’t mind making changes in electronics and perhaps cables (something all copper like Shunyata) and you recognize it will take time, then you will be proudly rewarded for your efforts with a dynamic, like live sound experience.