1) Are the standard Tungol 6L6 power tubes the way to go?
Pretty much. You wouldn't want to change them until your have familiarised yourself with the amp. Some have stated that the TAD 6L6WGC's are better.
2) I really like the tone of EL34s, has anyone become an EL34 convert?
I tried EL34's and didn't detect much of a difference in tone so changed back to 6L6's. Bigger change in tone (well dynamics really) with 6V6's
3) Can the Overtone take any octal power tube with the proper bias?
No don't think so.
4) What are the biasing range for EL34s and 6L6 in the Overtone Special considering the high plate voltages?
25mV to 38mV for 6L6's, not sure about EL34's though they are similar
5) Any favorite preamp tubes? NOS tubes in the Overtone worth it?
You might want to experiment with 5751's or 12AY7's in the pre-amp, it does change the dyanmics quite a bit. Currently I've a 5751 in v1 and a 12ax7 in v2, but I have also had 5751 and 12AY7. Not experimented with the PI or NOS yet though.