Dada do some excellent upgrade kits if you feel up to servicing yourself.
The 33 can be totally transformed IMO with the power supply revision and recapping elsewhere. I also found that replacing C400 (I think it was) on the output board with a 1uF cap (.68 fitted IIRC) takes the bass down to below 10Hz instead of brickwalling it at 30Hz and this opened up the sound no end on digital sources and allowed the sonics to "breathe," the results as big as the power supply upgrade IMO.
The thing with the 33, is that good original boards are available for not much money, so comparisons before and after are easy to do. I like my ton-up 33 and certainly am not looking for a 44. If the preamp is for a main system though, as said before, a LATE 44 will be fine...
The 303 is very much a triumph of its time, but the capacitor coupling and needed current limiting to 4A (to prevent the early ouput trannies from cooking themselves) can be a severe issue with modern 4 Ohm "Euro-Boxes." Give a fettled 303 a speaker it likes however, and it becomes an utterly charming little amp with a lovely midrange. I'm one of those who doesn't necessarily agree with souping the thing up to within an inch of its life with new boards, output transistors and front panel, but there are so many around still, that the choice is yours. The basic amp in good order is still a fine and charming, if small sounding thing.
Tear down these walls; Cut the ties that held me
Crying out at the top of my voice; Tell me now if you can hear me