i own a JoLida 502A that is heavily modified (see my bedroom system for details) - hardly anything in there is original. Without modification the 502A is a dark amp, has wooly bass (could be an effect of the used winged-C 6550 tubes) & slightly rolled off highs. Pretty damn good midrange. Post modifications it is a much better amp: tighter bass which seems to have much better definition. The biggest improvement is that it's much less dark. The high freq extension is much improved. So, after the mods, I realized that, tho' the used winged-C tubes could have been the culprit, the parts quality was a definite culprit. No doubt in my mind. The transparency of the amp is much improved such that it very easily shows the inadequacy of CDP (damn!). Alternately, when I feed the signal to it thru my CAT preamp in my main system, it sounds really very good! I had a hard time taking it out.
I personally feel that JoLida is a much better buy if bought used & modified. FWIW. IMHO. YMMV.