The gap in a combo head is too wide for full freq. resp. in playback mode. :>( Instead of simple cannibalization, how about a TOTAL makeover?
Perhaps a good playback only head can be found that mounts on the headbar. Turn the machine into a playback only unit that takes advantage of Sony's (IIRC) decent mechanical setup.
You said the O/P trafos look decent and the 6AQ5 is, for all intents and purposes, a 6V6 in a 7 pin mini bottle. Heck, cannibalize all the "iron" and make a NICE "flea" power triode mode SE amp. Triode wired 6V6 family tubes are "disgustingly" linear. ;>) It's your choice to use or forego loop NFB.
The 12DW7s test strong. Think about putting them into something like the tweaked RCA circuit I've uploaded. When those tubes finally wear out, the Sovtek 12AX7LPS is an excellent replacment.
Eli D.
