That would be correct, choose a primary impedance which is close to a third of a single pair P-P impedance. It doesn't have to be exact, just pick the closest available one.
The new K-16 amp from S5 uses the "reflection" trick to match up their existing 9800 ohm:8 ohm (35:1 turns ratio) to 5K which is what PPP quads of 6AQ5 / 6005s work into best. THey spec it as having a 4 ohm outout (though they say anything between 4 and 8 ohms will work). This "reflects" the 4 ohms back to the primary as being 4900 ohms.
The little UTK trannies will be straining at teh cloaca to deliver 16 watts, I'd SWAG that the freq response will lose an octav or two.
I decided to go with Hammond 1615s, a "true" 5K : 4, 8, or 16 ohms. with UL taps which may get used in a future mod / upgrade.
Warning, a little OT content follows:
I might just be putting some UTK OPTs up for pre-bay "adoption" soon, After this project, I'll have eight or ten of them "hanging around"...
They're fairly decent for 8-10 watts. a perfect match for a single pair of 6AQ5s or 6V6s. Oh, yeah, they'll work just fine with any of the 'BM8 or 'GV8 family, 11MS8s, a16A8s, 8B8s, nd even with PP 6EM7s or 6DN7s and their derivitaves.
I did a DCR measurement of the UTK's primary, it's 157 ohms P to P, 83 ohms on one side of B+ , and 74 on the other, this reflects the simple layered "solenoid" winding technique, these definitely don't have any "fancy" winding tricks...
The color code / phasing is the opposite of what Hammond uses, swap blue and brown around, if you need to use NFB.
/ed B in NH