I recently took the plunge on KT88s from EL34s in my ZMA and I am exceedingly happy. What you've already heard is true: lows go lower, highs go higher, soundstage is taller and wider; "sweetness" in the midrange is swapped out in favor of stability and realism. No particular frequencies appear emphasized or "hyped." Everything just sounds better across the board. (IMO; YMMV).

For comparison's sake: I was running RFT EL34s biased at 50 mA per Decware's ZMA manual. The RFTs sound really good; very sweet midrange. I am now running NOS GEC KT88s biased at 60 mA, per Steve's instructions via email shortly after I made the switch.

The GECs were kind of a holy grail tube for me, and I'm very happy with them. However, I tried two other sets of KT88s from different brands before settling on the GECs and the "KT88 effect" was in full force in all three quads. Not to say they all sound the same (they don't), but in my experience you're going to get the KT88 experience across the board so long as you get a dependable quad of decent quality valves. Many people seem to think Tung Sol is making the best affordable, mass-market KT88 right now, and the set I auditioned briefly gave me no reason to doubt that opinion.

If you do go the KT88 route, expect to give them 50 hours or so to settle in. I honestly didn't like the sound at first, thought I'd made a huge mistake and all that. It didn't take long for things to come together and now I only regret that I waited so long to try them out.

Nice on the GECs! It took me a few tries to find a NOS quad that didn't have any issues. But they're great.

I personally would not expect to find anything in another set of KT88s that you won't find in the GECs. I have a quad of KR Enterprise KT88s and they are very nice; I was able to swap in a KR or two with the GECs while I was dealing with getting a solid GEC quad. I did not notice any diminishment in SQ with the KRs in place, so I'm keeping them as a very solid backup quad.

Some of the Linai's did look interesting, I'll admit. A lot of decent Chi-Fi out there these days.

Regarding your "mortgage payment" comment: Yup! My first attempt at buying a full quad was indeed more than my monthly house payment. I justify the price to myself in the hope that the GECs live up to their potential 20,000 hour lifespan. Life is uncertain, but these could very well be the last KT88s I ever buy!

I ordered mine from Japanese sellers with solid return policies, just in case. (I find that, generally, Japanese sellers are very straightforward and easy to deal with as far as internet sales go.) It turned out that I did need to return two of the first batch, which worked intermittently but probably weren't safe to run. I think with tubes as old as these GECs, even if they've never been used and look fine, you just never know what they've been through as far as storage and handling over the years.

The nice thing about sourcing true NOS tubes is that even though I had to piece my quad together, they are all still reasonably matched due to GEC's high production and QC standards.

Started playing the GEC KT 88 tubes and my goodness! Only have a few hours on them and what an awesome experience!

Fuller sound, extension on the lower and higher ends, but ruler flat overall. No EL34 midrange bump. Initial thoughts are this is an incredibly detailed tube! I’d say it has lesser tube romance than my EL34 Mullard XF2 tubes, but have more oomph!

Love what I’m hearing so far!