Okay, sonics wise leaves a lot to be desired, but this is where the actual fun begins.
Began by soldering cathode bypass caps for the ECC99. 470uF/10V should be good enough but as usual, play with this value. I haven't spent time optimizing it yet. Now gain is higher and you don't have to crank up the pot like before. But I thought without the cap it sounded clearer. Anyway, let it run in first.
Next, wired in 1.2ohm resistors to every cathode of EL84 to check their current. Need to know how "matched" these tubes are. Well, what did I see?
My goodness! Matching is way way way off! One guy conducts 30mA, the other 50mA so cumulatively they measure 80mA but this is not right. We want each guy to be conducting 40mA, within a few mAs.
So, swapped around a few tubes until they match pretty well. If you are lazy and don't want to buy so many tubes, you should insist on matched tubes. This will make your life much easier. Alternatively, hang on first. A network to adjust current should be out soon. After I got my mix pretty well, fired it up and...
Sonics is sooooooo much better this time. Even had non-audiophile housemate jump at the improvement! Clarity is improved. Veil is thinner (is there such a description?) but there's still room for improvement.
YH's Lazy Man Constant Current Source
YH has suggested to me many times to swap the cathode resistor for a constant current source. But with me being LAZY, I abhor the process of finding suitable transistors (to conduct 80mA in this case) and all the works. Anyway, YH then showed me a pretty easy scheme! So easy you should knock yourself on your head if you don't try it!
After implementing the YHLMCCS, and verifying the current and voltages are okay (it works like a charm!), fired it up and... Non-audiophile housemate jumped higher this time! Veil is gone. Clarity is good. More amazingly, the BASS. Wham bang boom! Superb bass definition! Many DIYers have commented on how constant current source change their sound and better bass definition is certainly one of them. But hearing it myself is enlightening!
Sonics wise, this is something I could live with. But got to make it easier for DIYers to construct this amp. Proceed to the Update.