How did you go with the feedback issue?
My power amp mods in summary: finals as RD16HHF1s, feedback resistors R261/R262 as 820 Ohms, Transformer 2T primary / 3T secondary on BN43-202, 180pF across the output transformer primary and 330pf across R87/R88. That's all...in a way.
Heavily inspired by Erhard's (DF3FY) information as displayed on uBitx.net.
Results per band at 14V supply, RV1 at %60, RD16's biased at 250mA each:
Band output power Total current
80M 22W 2.9A
40M 20W 3.2A
30M 16W 2.3A
20M 16W 2.7A
17M 15W 2.4A
15M 12W 1.6A
12M 12W not measured
10M 12W 1.7A
At 22W the output transformer does get warm but not the LPFs.
Wound back RV1 to get 9-10W on the high frequency bands and 16-18W on the lower ones.
All the best,
73, John (VK2ETA)