i had an a21a for the best part of 10 years, a gem of an amp.
ok it,s only rated at 25 watts per channel but sounds more like 100, all down to the massive oversized transformer.
also dual mono, one transformer, but twin windings, each channel has it,s own power supply. the left and right boards are mounted directly onto the heat sinks at the sides....
bass to die for, sugden dont roll of the bass like a lot of other amp makers, it goes low.
dont forget it gets hot, i left mine on the whole time, no problems at all, probably best left on, it takes at least 30 to 40 mins to heat up and start to sing.. just make sure there is good air flow around and under the amp.
try the b & w 602 s3, discontinued but work very well, for the money they punch well above there weight, tweeter is excellent, not harsh at all. the bass with the 7" woofer is outstanding.
last sugden service is excellent, the a21a has been around since the early 90,s..
nearly forgot, there is a power amp version a21ap, same spec but will work with just about any quality pre...
enjoy..............a taste of the high end...cheap....
PS.
an a21ap power just finished on e bay for £275....pre from rotel or nad and sorted, bargain...you would have to spend a grand or more to better it....