Mark,
Having a Jolida 502A myself and have replaced caps and resitors, ect which I felt definely improved the sound of the amp. Nevertheless I think replacing the your pre-amp tubes(12AX7s and 12AT7s)will probably net you a better sounding amp (Jolida tubes ARE NOT the best sounding tubes you can have). I myself use NOS RCA in the 12AT7 postions and current issue Tung-sol in the 12AX7 postions. Switching out tubes definely change the sound of this amp, and it's one of the "fun things" about having tube equipment and it really doesn't have to be all that expensive. Truth be told, I have an assortment of current and NOS issue pre-amp tubes and I doubt I have much more than $100 invested in them.
Having a Jolida 502A myself and have replaced caps and resitors, ect which I felt definely improved the sound of the amp. Nevertheless I think replacing the your pre-amp tubes(12AX7s and 12AT7s)will probably net you a better sounding amp (Jolida tubes ARE NOT the best sounding tubes you can have). I myself use NOS RCA in the 12AT7 postions and current issue Tung-sol in the 12AX7 postions. Switching out tubes definely change the sound of this amp, and it's one of the "fun things" about having tube equipment and it really doesn't have to be all that expensive. Truth be told, I have an assortment of current and NOS issue pre-amp tubes and I doubt I have much more than $100 invested in them.