It was the output tubes all along...

I think I finally reached the balanced sound I have been seeking for years.  I've had my VAC 200 IQ's monos for about 7 years.  When auditioning the amps I was comparing these to a similar priced Pass solid state amp.  I liked the bass of the Pass, but really liked the tube sound of the VACs.  The VAC's come stock with KT 88's which seemed a little tame to me at the time.  Mike at Suncoast Audio was incredibly generous with his time during this process and rolled some KT-120's and then finally the new KT-150's over the course of a couple of hours.  The KT-150s added the bass slam that was equal to the Pass but had the nice warmth of the tubes to boot.  Sold!  Kevin Hayes was great-switching out the KT-88's for the KT-150s when my units were manufactured.  This was my first big boy purchase-a move up from an older MacIntosh unit.  We moved a few times and 5years ago I was able to build a listening room with optimal dimensions in what will be our last house.  By this time Mike had been able to secure lightly used Magico S7 speakers for me and things sounded pretty good. I've sequentially added rugs to the wood floor, first reflection point GIK art work, and other acoustic treatmens.  The bass was a little prominent, so I added bass traps as well as The Swarm subwoofers.  Things sounded great.  Then I added the secret sauce-VAC Master preamp with phono.  I was stunned at the sound I was hearing.  I added Valhalla 2 interconnects and speaker cabling with further improvements.  However, I started to notice the bass had become somewhat intrusive on about 30% of my records (mostly classic rock, jazz, blues).  I paid extra attention at live events to see what proportion the bass occupied in a variety of musical events.  About this time I found several excellent discussions on tube rolling here on Audiogon.  I paid extra attention to the posts of @Mulveling as he has owned most of my current equipment and has lots of experience with different tube combinations.  I played with some NOS input tubes which further improved the sound stage and overall timbre, but the pesky bass persisted.  It got in my head big time.   I stopped listening to the music, rather focusing on where and when the bass would vie for sonic dominance.  I reread some of the tube articles where @Mulveling stated he didn't care for the sound of KT-150s and found the best sound was achieved with KT-120s. I talked to Kevin Hayes who prefers the KT-88s.  What to do?  At this time I was sure I needed a tube change, but not sure where to go. I spoke with Mike from Suncoast, who actually had come previously just for a listen.  He liked the bass, but suggested I try the KT-170's.  He said it was the best sounding and tight bass he had heard and a has a set up featuring them in his shop.  I polled this group, but there was no one with an opinion on the virtues of the KT-170 over the KT-150 tubes.  Sooo. I am now 400 hours in with the KT-170's.   The bass is spectacular! Fast and full and but balanced. Didn't lose the slam.  No more subterranean rumbles, either.  The sound stage is broad with good depth and instrument placement. The mids and highs are great.  I had one audiophile friend over for a 3 hour listen.  He had heard the system before the tube switch and had commented after prompting that he didn't like the bass sound at all and it wasn't 5 minutes in that he exclaimed that the sound "was perfect".  I am now hearing only music. Using the retrospectuscope, as my system incrementally became more revealing, the faults of the KT-150 slowly emerged.  I see no detriment to any aspect of my sound after making the switch to the 170s, only a much much better bass.  Hopefully this post can help inform others as to what to expect with the KT-170 tube, especially if they are running KT-150s. I did check with Kevin at VAC who indicated there was no conflict with this tube, as the parameters were similar to the KT-150, but this may not hold true for other systems-so please check if you decide to try the KT-170's.  However, it's still not clear what tube is actually the best for this system...