your musical tastes can be provided by "tubes" but your loudspeakers are tough to drive with tubed designs.
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1. Most would agree that the Threshold Class A amps are sweeter and less forward than the Threshold Class A/AB amps. Albeit, at a cost. The Class A amps cost quite a bit more than the Class A/AB amps on a per Watt basis, cost more to run and run much hotter. Keep in mind that the Threshold Stasis Class A/AB amps run to about 20% of the rated power in Class A into an 8 Ohm load before sliding into Class AB. As the impedance drops Class A output will decreases and Class AB will increase. For example, a Threshold S/300 should offer 30 Watts per channel of Class A before offering and additional 120 Watts per channel of Class AB power into an 8 Ohm load, or 15 Watts per channel of Class A before an additional 285 Watts per channel of Class AB power. Depending upon your specific amp, speakers, room and desired listening levels you'll probably be operating at low power levels and conversely high Class A output most of the time.
2. The Threshold fet 10 two box combo pre will work for you. The phono aspect might have more to do with your prefered cartridge than the turntable. The Threshold S/300 is very compatible with most pre's that offer single ended RCA outputs. As has been said before Threshold and Audio Research seem to be popular matches.
3. Cable choices are typically chosen with regard to compatibility with the adjoining components and personal tastes. Threshold did market some hard to find speaker cables under the "In Concert(?)" label. Again many seem to prefer the Alpha-Core Goertz speaker cables (you must use the RC networks or appropriate Zobel substitutes with Threshold amps). The Gentleman that makes BMI power cords at one time had a vast Threshold collection, perhaps that's worth investigating. Cardas and Kimber seem to be popular interconnect choices. Again cabling is about fine tuning to your personal prefences within your particular system. Alpha-Core and Fat Wyre/UsedCable offer trial periods.
Have used Klyne, Spectral and Coda preamps as well as Threshold preamps with this amp. However the best of the group I used was the Threshold FET 1, which will accomodate two RTR decks. Although the FET 1 will most likley need servicing by now due to age. Also his first preamp the NS 10 will handle two RTR, although they are hard to find now and somewhat pricey now. As for me the best cables I have found were John Hilligs Superconnect IV and Discovery Cables. Both work very well indeed with Threshold units.
CORRECTION. Sorry, I left something out on my previous post, it should have read as:...."For example, a Threshold S/300 should offer 30 Watts per channel of Class A power before offering an additional 120 Watts per channel of Class AB power into a 8 Ohm load, or 15 Watts per channel of Class A power before offering 285 Watts per channel of Class AB power (INTO A 4 OHM LOAD)."
With the S300 the single biggest thing you can do is add a set of bypass caps to the two main filter caps. This will be bigger than a power cord. If I remember, the rail voltages on the S300 are 80VDC, so you would want to use 100VDC caps as a min. Try a 3.3uf, a .47uf and a .01uf in parallel on each caps. Parts cost will set you back about 6 bucks.
The S300 and S500 benefit greatly from parts upgrades, especially the resistors if Vishay bulk foil are installed, but that is a story for another time.