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

Your Dynaco ST-70 is a tube amp. While SS amps reduce output, as speaker system impedance rises, the typical tubed amp does not(given a correct impedance match/output connection). You might find this article enlightening. Especially, the comments immediately below graph 7, regarding output and distortion figures, into various loads. https://www.stereophile.com/content/dynaco-stereo-70-ii-power-amplifier-measurements Use your 16 Ohm tap, for best results.

Does your ST-70 still incorporate the 7199? If so- not much available, as far as tube rolling, now days. The EL34/6CA7 has never been known for a lot of dynamics, regarding bottom end. Sylvanias or Mullards, are what I had always kept in mine. They were much more plentiful(and- way less salty too), back then. No experience with what’s being manufactured now, so- can’t comment on their bottom-end performance. What rectifier are you using?  The Mullard(Blackburn plant)GZ34, is what I always recommend. Not that it offers any more juice, but- that it sounds so nice and lasts so long. I tried SS rectification(didn’t like it). You might contact Ralph and ask about beefing up your power supply(capacitance, etc), if that wasn’t already part of his refurb work. It’s been my experience, upgrading power supplies has always resulted in improved reproduction/greater dynamic impact, across the board.

If I still owned an ST-70, I’d be forced to purchase four of these(Did I mention, "salty"?): https://www.tubedepot.com/products/el34-mullard-double-o-getter-welded-plates I might be disposed to try the NOS, 1970s, Siemens version, toward the end of this page(and keeping a matched pair in each channel, if a matched quad couldn’t happen): http://www.audiotubes.com/el34.htm That page contains a lot of good info, if you’re going to shop EL34 types. How old are the Conns? Assuming you enjoy experimentation, here’s an easy/inexpensive way to enable you to try various NOS 6U8 type tubes(cheaper and more plentiful than the 7199), in your amp: https://tubedepot.com/products/6u8-to-7199-adapter Should you find a 6U8 tube, the sound of which pleases your tastes(Tung-Sol made some nice ones, in the 60s), it’s simple to move a couple leads on the tube sockets, obviating the adapter.   https://tubeworldexpress.com/collections/preamp/6u8a    Again- if the power supply capacitance is still stock(or aged), it’s still inadequate(by modern standards). Have fun and happy listening!

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.

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!