Hi Mike,
So here are some views not just mine but mainly from the dealer I auditioned an S500 from in Farnham and where I purchased my Snell J's in 1986/7 and the owner, Peter Qvortrup, of Audio Innovations in Brighton where I purchased my first S500 from directly. I did so because money was tight at the time and buying from AI meant I saved £200, the dealer knew but was fine about it. So about the dealer it was two chaps who were primarily audio fans and were just interested in getting a good musical sound, all they sold were Audio Innovations, Luxman, Snell and I think may also sold certain Tannoy speakers and some expensive large horn speakers, they were very selective and knowledgeable, I cannot remember what TT's they offered but only 2 or 3.
There are several design issues with the S500 detailed on the net, mainly some quality issues on some resistors, switches and the volume pot, but I suspect you have had these sorted as your S500 has recently be fettled. So based on the my suspicion is the valves will be holding it back.
If I recall correctly the S500 came supplied with Tung-Sol valves at the time, but I could be wrong. The dealer said that they were fine but more could be achieved with Electro-Harmonix, or the main recommendation for a purer tone and better control across the frequency range and bass was Golden Lion valves, they offered these as an upgrade at the time. I asked Peter Qvortrup about this when I purchased my S500 and he confirmed that the Golden Lions were very good in the S500, he also said that in his opinion running it in triode mode gave best results, and the drop of 6-7watts was worth it, he did this at no cost whilst I waited. He said Mullards offered a different tonality but some people did not like the richer tone and musical presentation, and were quite expensive anyway. So I purchased the amp with its standard valves, saved up and about 6 months later plugged in a full set on Golden Lions, these made quite a difference IMO as already stated. When I stupidly sold the amp I kept the Golden Lions and years later when I purchase another S500 they wen in it straight away.
So let's talk about the JJ Tesla EL34s. These are Chinese valves and are an attempt to copy the original Tesla EL34(Russian) no longer made, and NOS are very expensive, if you look on ebay 2 x EL-34 TESLA MATCHED PAIR BROWN BASE TUBES! NOS! 1960s are anything form $120-200. See
https://jacmusic.com/tesla/tesla-jj.html and if you read elsewhere the JJ Tesla's are generally considered to sound a bit harsh, can be unreliable and go out of spec quickly, so these may be hindering your S500. The same would apply to the E88CC's JJs. Real TESLA's are are different baby!
I cannot definitely tell from the picture what the 3xECC83s are, but possibly Tung-Sol Re-Issue 12AX7, if so then these are ok but can be bettered, if JJs then "While not as harmonically rich as others I tested, they do provide high gain without the usual noise and microphonic problems you would expect." and
http://6streetbridge.blogspot.com/20...cuum-tube.html as apposed to Golden Lion ECC83
"It is very smooth and pleasing to the ear. I attribute this to the broad midrange response. In audio or guitar equipment, grating highs and booming lows will give you listening fatigue or make you think your amp just sucks. The Gold Lion’s voicing is such that the midrange response seems full, while the highs and lows are nicely controlled. This tube would be great if you are retubing one of those classic old radio consoles." see
https://blog.thetubestore.com/review...on-ecc83-b759/
A full set of Golden Lion for the S500 will set you back £330 from Hotrox
and Electro-Harmonix (3*Electro Harmonix 12AX7 Gold Pin = ECC83) £66, 2* EL34EH £80 and 2 * Electro Harmonix 6922 Gold Pin - ECC88 £48 totalling £194
Personally I would go for the Golden Lion.
I hope this helps I have put the references to back up what I am saying, and so you know I am not talking out of my hat. I have also sent you a PM.