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

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!

Rodman, Thanks very much for the additional info. 

Yes, I will likely shop for a different set of tubes and check out the Mullards offered by Tubedepot. I don't know when the Conn 7199 tubes were manufactured or how many hours are on them. I bought them when I received it back from Ralph.  If needing to replace them I would likely switch to the 6U8 tubes after removing the two leads. The power supply caps have been replaced, however I will have to call Ralph to see if capacitance has been increased. 

I believe my ST-70 was manufactured in 1969. The first numbers of the serial number is 69 and I know it came from that period. I was in storage for most of it's years and is in excellent condition. 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. I am not an electronics buff, however did build a single generator as a kit when in high school electronics class. Worked from the start. Kit was much as the ST-70 one, just follow directions step by step. Seems the craft is in making a good solder joint. 

Getting a little long winded today, am having fun!

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!