That's a cracking system you have there! Huge fan of 6Ks, never had the pleasure of hearing them in a surround setup.

It's been my experience also that a full re-cap by someone who knows what they are doing completely transforms these older DSPs.

One other thing - have you replaced the tweeters? This has also transformed the sound for me on all mine. Since you have the skills, you could always replace one, and then compare with the other, as you have done with the cap replacements.

HC: 861v4/ID40/LPS, DSP5000, DSP5000VC, DSP3100, SW1600, HD621, Oppo BDP-105, Sony VPL-HW55ES
2-Channel: Oppo BDP-105, TEAC VRDS-7/Trichord Clock 3/PSU, 551, Quad 303, Harbeth Compact 7ES-3, Kimber cables, Russ Andrews mains and supports.
Previous Meridian: 5000C, 5200SL, 7200.2, 6000, 562V.2, 568.2mm, 800v4, 808.2i, 818v2/v3, 861v8, 502