What's the ECC99? I tried searching but to no avail.
I've found that decent tubes such as Cleartop or GE are difficult to get in the UK and wonder if in the long term servicing is going to be a problem. Hopefully, the 323S will be tolerable with vocals with the SR-404. Female vocals were irritating with the SR-407 and 323S and the combination of the SR-404 and 006T seem to reduce this problem. My torture track (which caused me to begin to hate my Sennheiser HD700), Slow by Kylie Minogue which is usually sibilant, is acceptable with the current Stax gear.
Is it normal to hear a crackling noise in the left earspeaker when the 006T is cold, say a few minutes after turning on? Oddly, with an MP3 playing with the 006T set to zero I can still hear sound so this should be fixed on servicing.