Hard to say, I can think of the following spec highlights for comparison, and you'll be the judge:
A51 has better numbers in the following:
- THX Ultra2 certified
- 2.2 kVA encapsulated toroid power transformer with independent
secondary windings for each channel
- High bias Class A/AB operation
- 164,000 μF power supply filter capacitance
- Slew rate: 130 V/µsecond
- input impedance 47 k Ω unbalanced; 94 k Ω balanced
- ACD 250 W/400 W, 8/4 Ohms, 0.2% THD at full power
- Damping factor: 1100 at 20 Hz
MCA525 has better numbers in the following:
- 600 W rated into 2 Ohms at <1% THD
- SNR, IEC-A, ref. 225 W . . . . . . .120 dB Vs A51's 112 dB IHF A-weighted (Note: no idea what IHF is, probably an old standard, so this may be be an apples to apples comparison).
Note1: A51 offers no 2 Ohm rating, though the slightly more powerful two channel version, the A21+, measured 620 W at 1% THD, both channels driven, and into 2 Ohm, 900 W with one channel driven.
Note2: The MCA525 specs did not include any ACD output rating, or its there and somehow I missed, may be
@lovinthehd can help fill in the blank again.
MCA 525 has worse numbers in the following:
- Slew rate: 30 V/µsecond
- <1% THD at rated output
- Channel separation, 100 Hz to 10 kHz >65 dB Vs A51's Inter channel Crosstalk > 78 dB at 1 kHz > 63 dB at 20 kHz so I would say the A51's a lot better based on the typical shapes of XT vs freq., that the MCA's would likely be a few dB worse at 20 kHz. Both, based on specs are no better than the AVR-X3700H, SR6014, and much worse than the X3600H measured by ASR, but I guess only golden ears know the difference in use.
- Input impedance 10 k Ω unbalanced; 15 k Ω balanced
- Damping factor: 300, 20-1,000 Hz (Note: 300 should be more than high enough)
I would say the A51's specs would impress audiophiles more, whereas for practical use, the MCA525 is probably better because of the much lighter weight.
To me, it would be a tough call, good thing I already have a MCA and Halo amp, albeit the older two channel version (MCA20 and A21).