Amplifier design parameters required for a good match for high (16+) ohm speakers?

If you can solder and read simple schematics, improving the ST-70 is cake. There’s a wealth of info available. Just restrict yourself to those best reviewed, far as advice(ie: Ralph). Joe Curcio gave me some valuable tips, on how to increase my Joules, without adding/changing anything outside the chassis.  Regarding that wire swap, you might want to save this: http://lib.store.yahoo.net/lib/thetubestore/6U8A-7199.pdf Don’t know if Joe still offers free advice, but: http://www.curcioaudio.com/dynadr_3.htm#DYNACO%20STEREO%2070%20POWER%20AMPLIFIER           Was that a Heathkit Mod #IG 5282, in a blue plastic case? Wasn’t my first build, but it wound up working for years, testing speaker drivers in my shoppe.

Rodman, Thanks for the additional info!  I have taken a brief look at the additional links. I intend to peruse all the links provided again and possibly set up an upgrade project with my ST-70. Won't happen in the near future however, other things on to-do list. 

I don't remember the signal generator model. Built it fall of 1967. 

Thanks again, you have been most helpful!

Your welcome and I hope some of it’s useful(I try). btw: I found this power cable will fit the ST-70, with a rubber grommet and no need to further open the chassis’ original hole(after removing the strain relief, of course). I crossed and closed(without crushing) a couple cable ties around the cord inside, to prevent it’s escape(kinda Mickey Mouse). A snap to hard-wire, internally. It made an audible improvement, though I went cheap on that one(an experiment, at that point). No doubt, there are better cords now, just as small. https://www.zuaudio.com/classic-cables/birth-power-cable There’s an inexpensive/NOS one, currently on eBay: https://www.ebay.com/itm/Zu-Audio-Birth-power-cord-2m-6-5-ft-Brand-New-Factory-sealed/113672669785?h... Happy listening!

Ralph said it was in the best condition of amy he has seen for that time. It was factory manufactured, not put together as a kit. I own a original owners/instruction manual for the kit version so following it, I could likely replace the power supply caps myself if I feel the need.

If it came though here, then we replaced the filter caps. The ones installed allow the 5AR4 to be used, but do have a bit more capacity than stock. We also replaced the bias supply filter caps and rectifiers and probably the coupling capacitors too- the amp was set up to be reliable.

Ralph, I am using the 16ohm output with the Contrasts, which are rated at 16ohms. I am not sure about actual impedance within the bass frequencies.

Which model?