I have one - an MP-501 V5 - which I purchased in July 2019, during a sale in which I saved $200. I already owned an MP-301 V3 and was, and still am, very pleased with it, but the 501 is a dramatic step forward.
I bought the 501 because I wanted more power than the 301 could provide and I wanted to relegate the 301 for use with headphones. There is no doubt that MP kit is much cheaper than most comparable valve/tube kit, but I simply could not afford valves/tubes otherwise. At least with MP I can afford to buy brand new.
The 501 arrived fairly quickly from Hong Kong (MP is based in Canada but the amps are clearly manufactured in HK or China. The packaging was superb and the amplifier looks beautiful. I don't think there is a another KT120 amp available at this price which comes from a reputable manufacturer.
I am using KT120 valves/tubes (KT150 are available but they are rather more expensive). The amp comes with 4x glass 6J8P pre-amp tubes; these are Chinese copies of the American 6SJ7 and they can be bought very cheaply on eBay, new from China. I have plenty of spares in stock - probably enough to last my lifetime. People online carp about these valves but I think they are excellent. I also have a large stock of RCA metal 6SJ7 and RCA metal 6SH7 and have tried both in the 501. At present I am using 6SH7 for V1, 2 and 6J8P for V3,4.
Using a MacBook feeding a Chord Qutest DAC, the sound is magnificent. It is a substantial improvement on the 301, which is itself excellent. The separation of instruments is far improved. The bass is very much tighter, deeper and more realistic. I am very pleased with the amp.
I hope this is helpful.
