Based on what I have read, you must avoid the "A" version and go only to the original 6600 version (the "A" version is not as good as the regular 6600 version).
These monoblocks are known to have some reliability issues, but can be fixed if the genuine transistors are used (versus counterfact transistors).
I have read that they sound good but do not like the low impedance speakers.
Hope this helps
