MrAqua, your question is really a good one. As I see it, in changing the speakers to an 8 ohm load, the need to replace the output transformers is really important. I believe that Dave pointed out rather early in this thread that the 'impedance mis-match' between the 4 ohm output transformers used on the 8600 series amps, and the 8 ohm speakers used by most folks in their home audio system....this rather 'robbed' the amp of a significant amount of power alone.......like about 50% or so. So, if you're switching to 8 ohm speakers, making the change to the output transformers is really a good plan. Because of the change in secondary output impedance, the changes made on Dave's design for the 'Feedback' section should probably be included as well. So, you'll probably want to find the old feedback circuit, find what is probably just a single resistor, and replace that with the 10K / 300pf network that Dave designated. But, beyond this, here's the rub: Dave did this project with the intent of redesigning this amp as a 'stand-alone' power amp. He did some calculating on current capacity, and in LOSING all the peripheral sections for radio/pre-amp...... the power transformer 'could' supply the needed 100 ma of current when built as per his design. But, you're keeping all that stuff, right? So, if you attempt to re-work the output section to the way Dave designed the bias, you'll be significantly increasing the current draw from that section, as well as retaining that from the other sections. This may prove too much for the power transformer itself. So, it might be best if you simply left that section of the amp alone. The addition of the Zobel network might also be considered.....that being the 47 ohm resistor and the .1 / 100V capacitor coming off the output side of the output transformer (seen in the circuit as connected to the Feedback line). This may prove beneficial in reducing some potential distortion. But, beyond these three changes (Output transformers, Feedback network, Zobel network), I don't know that you'll accomplish much by doing anything else. You could play around with the bias section.....and 'maybe' increase the bias load some, but you'd risk putting too much current draw on that transformer. My humble opinion is that beyond those 3 things listed any further changes will likely not provide any real 'audible' improvement. If you get really ambitious, you might consider re-wiring the 6EU7 socket to accommodate a 12AX7 tube. 12AX7s are a LOT less expensive than 6EU7s!! But, that's a bit of a challenge in that little amp! I've done it twice, once because I had to....the old socket was shot! I might add that if you DO decide to do this, replacing that tube socket with a better one is also a good idea!!
OK..........there's my $0.02 worth! Hope it helps give you something to think about. Other's may feel differently or have 'additional' suggestions. So.... good luck with your final determinations.
Tom D.