Hi all,
In answer to the above, yes the vol pot greatly affects gain and is pretty poor.
My 708B stopped working a month or so ago. I noticed that the output phono connectors were badly corroded! (gold plated?) When I opened it up, the 6z4 had dropped through. 2 of the soldered terminals were broken. I just re-soldered them, removed the wire tube clips(the cause), and fitted quality phonos. All working again!
Upon reading this brilliant thread, I was inspired!
Here are the mods that I have made so far.
1-Changed the 6n3 for a russian 6n3p- more gain, clarity, detail.
2-Changed input wiring to ofc/ silver plated- again, more clarity, detail, much improved treble.
3-Changed 2 rear valves to russian 6n23p- at first very bright, but after 2 hours running became very smooth, with typical old school valve sound. Fab mid range and treble.
4-Changed vol pot to Audio Note(100k), ofc/ silver wired output, replaced output resistors with Kiwame. Now sounds very smooth, detailed, holographic. Treble is all there, but clean. There was some distortion in mids/highs prior to this part of the upgrade, now totally gone. There was a big drop in gain with the Audio Note pot, but now more realistic. The old pot only hit about 8 o'clock to give high volume. I kept the original knob by using next size up drill bit to enlarge the hole size. Had to file the front panel hole to fit. It works great and all looks original.
The amp just sounds amazing. I am using a high end Alchemist DAC with Audioquest Diamond and a pair of stunning Beyer DT990 600ohm. Each upgrade has been very easy to hear. I am looking at changing the output caps next.