I would love to know what the difference is between the S300, the S300 (series 2) the S300E, etc. Also, can anyone suggest the best pre-amp to use with the S300? Any information would be greatly appreciated. Thanks much.
There is some kinda version that is what they call Optical Bias... It basically was a better way to keep the amp within certain power range (keep it from floating) when your outlet power from the house would fluxuate and kinda make the amp not sound so great... More or less keeping your transistors all within a certain current and temperature for optimal operation. This I believe was the series 2' but they made some series II without the optical bias circuit early in the series which at the time from reviews and used sales standpoint the optical version is a much better amp than the series one or Series II without it.

I owned the series II Non-optical and honestly it was big and pretty box but never worked out for me, other thresholds were much better sound. The series 2 versions were a lot harder to come by in the used market with optical bias and sometimes quite a bit more money like 500 bucks. For example you could buy a used S300 for about 900 bucks, but an optical bias could go up to 1400 even 1500 used, original pricing was the same I believe somewhere between 3200 & 3600. All of these amps are pretty old now so they can use work and capacitors, as seen in your other post you can get a hold of Jon from that vintage amp repair. Oh and if you have the optical Bias it says it plain as day right on your panel by the power switch and its also possible that it could have been upgraded to optical bias in which you would not see this on the faceplate, but not sure this was possible anyway.

Also these amps were pretty prone to transformer noise, where you listen to that big case and hear the transformer buzzing or even rattling around.. Sometimes a quick fix tightening or deadning it could help, but I am sure there is some other tricks... One I had was a s200 or was it a s150 which was my favorite smaller sounding amp needed a new transformer when it developed this issue from some kinda delamination...

As for the 'E version not 100% sure accept most of the E series from them were upgraded to pure CLass A opposed to the standard A/B operation, so like the SA4e type amps etc...But these amps are VERY hot when running. However they were also much more expensive and better sounding amps.

Good luck
Undertow summed it up very well. The series II is said to be less dry at the top and tighter at the bottom than the earlier version. The e series, unlike the previous versions are DC coupled, true balanced, and in the stereo versions, dual mono with seperate trannies. Some claim that the series II is sweeter at the top than the e series, but, the e series is even tighter at the bottom and more vivid and transparent.
Gentleman, this is great information for me. Any other comments or important information would be welcomed. Thanks much. I look forward to all other comments.
If you want to stay with Vintage I ran my Threshold S/300 series 2 with a Audio Research sp11 pream and it was a exceptional combo...Also has a great built in Phono section too....
Threshold's own fet10 combo will work very nicely. Audio Research and Sonic Frontiers (though I think with the S300 many of the benefits might be lost)) are reputed to be good tube matches. FWIW, Audio Research is also reputed to work well with Maggies. If cost is an issue, the Forte pre's might be worthy of consideration. I'm guessing that with the e series, if you want to use a tube pre, it might be prudent to stick with one of the few DC coupled tube pre's such as Atmasphere or Sonic Frontiers.
I run the S/300 with an Audio Research LS-2. It might be a good idea to have the S/300 checked - it's around 20 years old. Some caps might need replacing, the bias might need adjusting etc. No big deal the 300 is built as a tank, but some components do age.
I am most grateful to all who commented here. I do appreciate your great advice. It will be very useful to me as I look further into this awesome audio area. I cannot wait to move into my new digs and get everything set up and start listening to great music. Thanks much.