Better amp or better tubes?

from my experience with kt90's kt88's/kt150's. there is only lsight gain in sonics over each, the 88's beat out the 90's, the 150's beat out the 88's, not by much, IOW I'd be quite happy with 88's vs the kt150's i bought at 75/each. x 12.
also i note, a  88 is a  88 is a 88. The sonic difs in power tubes is so ~~miniscle~ as my baton rouge tech guy uses that word,,, = all 88's are equal. 
 but now in 300b tubes there are some incredible looking, but expensive designs, = the mesh plates etc.
hummm, tough call. 
if i were you i'd sell the QS and go with a  new 300b amp, then later on upgrade the stock 300b tubes to mesh plates. Line Magnetic anda  few other chinese labs have interesting looking designs. for a  very reasonable price. My fav chinese lab is Cayin. Made by Spark. they offer a  stunning 300b design. 
hope that helps.

treat a tube like any audio component. you must know where you are tone wise and where do you want to go from there. more bass and warmth? more highs? maybe you need a bit more output so higher gain power tubes can do the trick. any brand and model will give you something else so you should start learning about the types in your amp ,what's available and how does it sound,and pick the best for your taste . asking other 300b or quicksilver amp owners can help. worn tubes lose highs,if that's what your missing a new set of the stock  tubes will work. in a handwired circuit your preamp/input tubes are your preamp,your power tubes are your power amp .i advise you to buy the best nos/nib power and preamp tubes that you can afford,from a known seller that accepts returns,it won't be cheap but you will get better sound and wouldn't have to worry about it for 20 years. the technical part is testing and swapping the weak ones the audiophile part is just like picking any component as an upgrade. good luck!

since someone suggested it let me make it clear: n e v e r buy nos tubes thinking to keep them as spares if they won’t sound good. if they don’t sound good you picked the wrong tubes,it takes 20 seconds to tell on a set you live with. send them back! get something else or your money. never buy without the option to return if you don’t like the sound. if you want to tube roll just to get an idea ,buy used tubes for cheap.
 there is no excuse spending this much for audio and not getting the best.
" The genalax 300b are modest in price" sorry mate but not true. your stock tubes are russian made and still costs 300$(maybe 50$ maximum to produce...) .since there are only few that make 300b these days,and nos 300b prices are so high that they can get away with it ,they are priced so high for what they are. so what? let the lemmings have them.
the best thing to do is to ask mike sanders what tubes will sound the best on his amp.

Curious as to what type of change you are looking for in your sound?

During all of my Covid-19 restlessness I have sold a Primaluna Prologue Premium integrated, Evo300 integrated, Rogue Audio Sphinx V1, Schiit Yggdrasil DAC.....all to acquire a First Watt F7 (used), LTA MicroZotl preamp, McIntosh MCT450 transport (used), and a Timbernation rack to house all of it.

So, I’m feeling your pain!

I have battleships from the 60’s, they are awesome, 25 years is young for the equipment, but a long time for tubes. And, the old stuff has lots of room and access to repair/replace anything in the future, the newer stuff is crowded inside.

1. re-bias existing tubes, assistance from Quicksilver, listen, any change?

2. I would definitely try new tubes. I have read a few articles that indicate NOS is not always best as assumed. I would try less costly version, DON"T LISTEN TO THEM. break them in 60 hours (don’t need to listen, just leave them on, walk away). Break in is a real thing, avoid a bad first impression prior to break in.

3. Then, put originals back in, listen to compare with new.

4. Verify with Quicksilver how to bias the new replacements in case some minor difference might effect things (probably the same, just eliminating any potential for a false comparison)

5. Listen to new. Nirvana re-discovered?
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Brent Jesse is a terrific source.

http://www.audiotubes.com/audtube.htm#aud%20list

NOS WE $3,100. each, based on rarity.

New current JJ $150. each, constructed like WE, ceramic base which I like.

I would buy matched JJ’s, and,see what change.

Then, if you have decided to keep the amps, and you have deep pockets, and have to know, Brent will let you return the JJ and buy the horribly expensive NOS. Confirm that with him ahead of time, if NOS not better than JJ, I would switch back.

Other member said "a 300B is a 300B". I certainly found that not to be true

my recent 6SL7 replacement. Original Cayin brand, unknown maker, sounded terrific. One side tube went bad. I have 2 tube testers. I had good used GE in stock, put it in, wonderful together with the used Cayin,

So, let’s get a new matched set. Picked Sylvania 6SL7WGT military from Brent: nooo! original used Cayin and used GE definitely preferred. Brent let me finish breaking them in (they did change a bit, which is why I advise breaking new tubes in first before listening.),

and eventually switch them for a matched pair of GE’s like my used one. (I got 2 matched pairs in case I live that long), at half the price, and Brent refunded the difference.
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And, I was using 6550’s, liked them, just for fun tried KT88’s. I prefer the KT88’s slightly.