Cary SLi80 amplifier - Effect of power tube exchange

Several months ago I purchased a used Cary SLI80 Signature amp that I’m using with Martin Logan CLS2 speakers. The sound was very good as received so I put off experimenting with tube selection or  checking bias adjustment. Because, though, I was able to buy a matched quad of Gold Lion KT88 tubes at a very good price, I decided to try them.  I switched them with the stock Electro-Harmonix tubes and measured the bias amperage. I was surprised to find that one channel was reading significantly higher than the Cary recommended setting  of 75 mA, at 85mA and the other channel displayed a lower value than specified @ 50 mA. I hadn’t measured the bias amperage before making the switch so have no reference values for comparison with the original tubes.  Also, I don’t claim to have the best ears on the planet but the difference in sound was obvious.  First, I didn’t expect power tubes to impact sound much and, according to things I’ve read, the direction of proposed potential change was opposite of that predicted. Upper  frequencies seemed more defined and forward especially with piano music which comes across a somewhat strident.  This is most noticeable when set in ultra linear mode. Maybe the overall change is toward improved accuracy and I just need more time to adjust my listening to it but I’m curious to hear what comments that those of you with more experience have.  Are these changes the result of different tube manufacturers, bias adjustment or, most likely, a combination of these factors?  At this point in my thinking, though, the obvious next step would be to return to the original tubes with proper biasing and reevaluate. 

75ma and the Gold Lyons and you should be GTG. Give it a chance before you do something different. 
You're half way there.  After the GL88s output tubes settle in, changing out the two stock 6922 Input Buffer Preamp and two 6SN7 Pre-Driver/Phase Inverter tubes makes a notable difference yet again on the SLI-80.   Sure enjoyed mine even more after tube and cap upgrades.    
In the owner’s manual Cary recommends staying with the Electo-Harmonix KT88’s but I’ll give it a few more days with the Genelecs to see if I’m the factor that needs breaking in. This is my second Cary amp, the first being the Rocket 88, and I’m convinced that for tube amplification, they would be hard to beat for my listening taste. Please understand that in no way do I think it sounds bad with the GL’s; it’s just different. BTW, my sources are Ammara, Amazon Echo or CD (no longer any Vinyl) so I’ll also experiment with the use of several of the DACs which I have for use with Ammara and the CD transport (the Echo provides only an analog connection). A fleeting thought that I had was to try replacing the rectification tubes with solid state diodes but, because speed is not a factor, at least to the extent that I understand it, I decided against it. BTW, Decooney, the previous owner of the Cary had already recapped this unit so I’m good to go in that regard. 
The sound will change a bit until those GL88s fully burn in. Give them some time. I enjoyed rolling several different new-reissue and vintage NOS 6992s and well matched 6SN7s in mine.  That amp respond well to really good tubes.  

Keep in mind the ML CLS2 speaker is 86db sensitivity and 4ohms, so your SLI-80 is working a little bit harder if you turn it up. You might see a bit more dynamics in the ultralinear switch mode setting with your speakers. Good fun.