Audionut:
You need to do a MUCH better basic SET amp circuit than what you are thinking of doing, if you want the best possible 604 results.
Sub woofers are not ever needed, in my experience, with 604s.
People add sub woofers because of two primary factors, (1) their entire systems' wiring and its resultant transfer-efficiency-of-energy, is simply not good enough. (2) They use conventionally executed SET amps, with all their WOES, as I have described the last month.
AMP WOES, of SETs used on ALTEC 604s :
ie : The AMPS use chokes over 20 Ohms in DCR, too much capacitance in the filter to the Finals ( robs dynamics as explained by EE, Hasquin ) , use more than two tube stages, and generally, SET amps typically suffer with a VERY poor internal transfer efficiency of energy, ( unconsciously ) through the use of ineffective internal wiring and lay-out.
ALL the above factors, when mis applied to amps, and to ALTEC crossovers, ruins their potential performance, in my opinion and experience.
Some "add tweeters" because they do not know how to multiple bypass the cap positions in their SET amp and speaker crossover, while their SET AMP plays like 99 % of the SETs, narrow band, non-dynamic, and primarily in the midrange.
The best ALTEC system "top ends" I have ever experienced, come from SETs ( and speaker crossovers dudes !!! ) that are multiple bypassed, in every cap position, EZily !!
Mind you, boys and girls, this is all "IN MY OPINION", and...... it is also in my experience !!
I started listening to an original ALTEC 604 in December of 1944, after my Mom brought me home from the hospital.
I hope this Forum is broad enough, for me to express my honest opinions. My intent is to inform, not to offend others. If you follow up on my suggestions, you will discover I am not too far off base at all !!! Have FUN, I DO !!
Dowto1000