I own a JoLida 502A & bought it over the 302A or B 'cuz (altho' the EL34 tubes have a magical midrange, the power o/p was unlikely to be sufficient for my needs) of the 502A power o/p.
>>"The pro shops offer an upgrade that does little more than change 4 resitors and 4 tubes. Of course they are not forthcomming with that info."
Not true! Either you are not looking diligently or you are but are discounting what they tell you. For example, see this website: http://www.responseaudio.com/jolida%20signature%20series.htm
Anyone who reads this will agree that there is a ton of info on this page & that if you are handy with a soldering iron, you could look @ this page & do the mod yourself!!
Anyway, I think that the EL34 ckt can be modified to use a 6550/KT88 o/p tube. I once knew where to find this bit of info on the Response Audio website but seem to have forgotten! If you poke around the above site, you might come across it. Otherwise, email Bill Baker - he's easy to communicate with over email & quite forthcoming.
Just FYI: I tried my 502A with my 89dB efficient speakers this past weekend. Results? Bad!! The volume needed to be around 2 o'clock for reasonable listening & 3 o'clock if I really wanted a little more SPL 9' away for a favourite track. That left very little headroom for dynamics as a result the music sounded rather 2-D. Now, I do have 6550 output tubes => I probably have 45W/ch. The music would probably sound better if I used KT88 tubes. I agree here.
With my 94dB efficient speakers, the JoLida with 6550 output tubes is a pure treat! Completely different animal with a great sound. Let me add that I'm not 1 bit surprised at this outcome. One speaker being 5dB more efficient means that it demands between 3-4X less power as a result the volume control is at 9 o'clock (max)! Plenty of headroom for dynamics.
So, just confirm that the 302 or 302-converted-to-502 is the right amp for your speakers. Otherwise, you'll be wasting your money.
With the KT88 you'll get the rated 60W/ch which might or might not be enough for the type of music you listen to.