The MZ2 is only one watt into 4 ohms and the A21 is 400 watts into 4 ohms. I think the MZ2 amp section should be used as a headphone amp only, or a pre to virtually any other amp. I say "virtually any other amp" because the output impedance is only 2 ohms and should match with any amp regardless of the amps input impedance.
I put about 120 hours on the MZ2 amp section and still found the sound thin. I see real potential if it had 20 or 30 watts (or more). Even though the sound was much thinner than the A21, it still had good air and space. The sound signature was definitely different - in a good way. It was what I expect to hear from a good tube amp. The potential is there but the watts were not. At 12 o'clock on the dial the MZ2 was putting out about 70 dB. I would definitely consider the more powerful LTA amp offerings if I were in the market. I would love to hear shootout between the A21 and ZOTL10. Hope this answers your question.