What tubes for sibilant SET Triode amp

I have recently bought a Cayin/Spark 9021DEX amplifier. This is a zero feedback SET design in which I currently use the original Chinese made 211 output tube a 6SN7 GTB by Sylvania as a driver tube and a late Siemens ECC88 at the input. The bias on the output tube checks fine the sound is great with a minimal tendency to sibilance. As I am going to buy a set of back-up tubes anyway - which tubes to buy to get rid of the tendency to sibilance? Opinions seem to vary considerably! Any suggestions??

Preamp is a MacIntosh C220, Speakers are Gallo Ref 3.1 and Analog Front End is a VPI Superscout with a Shelter 90X.
I replaced my ecc88's with Amperex tubes soon after I bought my preamp. I didn't like the Siemens tubes at all.

I agree with Dave. Many (maybe most) ecc88/6dj8/6922 are microphonic even when they test well.
So your amps make about 12-15 watts- that means the Gallos have to be about 102 db to really take advantage of the amplifier's capabilities, and I don't think they are anywhere near that! So my guess is you are pushing the amp too hard to let it show off what it can do. Instead, you're getting a lot of distortion out of it (sibilance). I think you will find that if you go with a more efficient speaker things will change a lot. SETs in general are not known for sibilance!
Roomsize and listening preference make me listen at 85dB max.most of the time..so even with my Gallos rated at 89 dB - there seems to be plenty of headroom for my amp that is rated 25w. And in fact it can go MUCH louder than I care for before distorsion. Additionally,an old Marantz MA5 with power leds indicates that I usually listen with about 3 Watt max. So my guess is that I drive the Cayin well within rating. Again the sibilance is slight and just a tad more than I care for and a tad more than I get with a Triode Corporation KT88 push pull rated 35 W.

Microphonics: the Siemens are fitted with EAT cool dampers. But that may not help. Thanks for the suggestion.
How sure are you that the tubes are to blame? I wouldn't advise buying some expensive NOS tubes to cure this problem until you know you've isolated it. Buy a couple of cheap JJ Tesla 6922's and try those first.

A couple of other suggestions (I know they sound stupid, but I've done all these things myself)...

Make sure your speaker wire is connected to the TOP binding posts on the Gallo's.

Make sure the top and bottom binding posts haven't been accidentally or intentionally bridged together.

Make sure you didn't reverse phase when you hooked up the amp. Is your new amp phase inverting? Find out?

Has your amp had time to break in?

Is your tweeter set at -2db? Set it to 0 while you get to know your new amp.

Start by changing the input tube.

If you want a rich/full 3D sound try Amperex solid dimpled disc getter 6dj8. Mullard made one as well (with a "G" factory code I think).

Slightly less full, but just as good is the Amperex Orange Globe label 6dj8.

These are much less expensive than similar sounding e88cc's and they work fine as an input tube (used them in an Audion Silver Night 300B).
Some people have quiet environments and their rooms are lively at the same time; I can see how that can work. Tfkaudio's suggestions sound like a very good place to start. Like I said before, sibilance is something SETs are not known for, and a bigger one like yours even more so.
I didn't realize that you are Switzerland based.

Add Philips Miniwatt 6dj8 to the list as they should be fairly common on your Ebay.

I'd also like to note that using Siemens 6922 types in my old Audion (300B/5687/6922) amp caused sibilance, therefore my experience differs from that of Ralph @ Atmaspere.

80's Mullard CV4024's (used as drivers) make my current 2A3 SET amps sound like a cheap transistor radio. Telefunken 12at7's make them sound too bright.
Thanks for the couple of great hints. Gallo sides I checked all connections; these seem fine. The Cayin do not seem to be phase inverting as they collaborate well with the Gallo subamp also hooked up to the Ref 3.1. In fact I get +/- 3dB all the way down to 25 Hz with a small dip around 60-80Hz of -6dB. No possibility of adjusting treble on my speakers though.

It looks as if the input tube should be the place to start tube rolling. Likewise I will experiment with replacing the Nordost Red Dawn speaker cable that I currently use by something slightly less forward.

Sorry - just got the itch that there could be just that little stretch to be covered to achieve perfection (knowing that I never get there anyway....)
I found the Telefunkens to be the smoothest yet still detailed 6DJ8 variant around