More bass from Cary SLI 80. Tube changes?

What can I do to increase / refine the bass output from my Cary SLI 80? Tube changes perhaps? Which tubes would be the best to increase bass?
Talk to Andy at:

Re-tubing my preamp was the best performance/dollar I have spent on my system. I bought NOS telefunken and RCA tubes from Andy. After 2-4 weeks break-in I never looked back.

I purchased Cary SLI-80 a couple of months ago. One way to improve bass is to try various interconnects and speaker cables. The SLI-80 gives a tight and powerful bass when used with the Straight Wire Virtuoso R interconnect. This cable requires a long break-in period and must be thoroughly matched with a CD player. It has proved incompatible, for instance, with my otherwise excellent CAL CL-15 player (balanced version) but has shown good results with the Cary 308t CD player (especially with jazz and classics). Replacement of the stock 6922 Sovtek tubes is a must. NOS Amperex 6922 (globe logo) works fine, not in terms of bass, but in terms of overall focus, timbre and soundstage. You may also try NOS RCA 6550 and 5U4G tubes. I bought the RCA 5U4G rectifier tubes and am quite happy with them.