Better amp or better tubes?

From your post, it seems that you have decided that the tubes and/or the amp is what is causing your desire to upgrade.  I think you should look at your whole system.  I recall once going into a dealer absolutely convinced that I needed a better or more powerful amplifier, which he could have easily sold me.  But he demonstrated that upgrading the preamp gave me much more of what I thought a better amp would do.  Just something to think about in these troubled times.  

Hi Chayro
Recently done just that. Sold my LTA Microzotl2 for the LTA Preamplifier. VERY happy with that move.
So my attention has turned to the amp.

Troubled times indeed. We all need a distraction

I would test the tubes to see what condition they are in. Its not surprising to find that a good quality 300b has lasted 25 years; they lasted over 50 years in the intercontinental telephone trunk lines which went from the US to Europe under the sea.  But the other tubes in the amp might bear taking a look-see.

I think you should look at your whole system.

This^

Take Atmasphere's advice. Freshening up power tubes can make a large difference. Just did that on my amp - same type/brand. Tightened up the bass added gain clarity in the mid range and restored the highs. The worst possible result is you will have a spare pair of tubes to hold in reserve. 

FWIW MC's advice is, IMHO, an invitation to audiophile angst.

Although they were expensive, the 1990's Western Electric reissues are extremely durable; I know a number of people still using those after 20 years of use.  A lot depends on the design of the amp.  If it does not push tubes very hard, they will last a long time.  If hte designer tries to chase the last bit of extra power and pushes the tube near its specified maximum, life is dramatically shortened. 

 This is particularly bad news for amps running 300b tubes because there is no such thing as cheap alternatives.  Once again Western Electric reissues are being offered, but, it is not possible to know if they are as durable as original tubes or the 1990's reissue.

@emil emil,
What do you NOT like about the current sound -or- is it just curiosity / upgrade-itis?  

Other factors that impacted my QS amps negatively or positively, assuming your speakers are good, there  are other things to review too:
-Your preamp (done), LTA very capable. Check that box for yourself.
-What coupling caps, "shhh", did Mike keep in your amps (key)?  
-Your interconnects, are they veiled over or transparent type? 
-Your input tubes, which ones are you using, brand/type/year (key)?
-Your speaker cables, brand, type, length,std or bi-wire? 
-Source: vinyl or digital, DAC, type?
-Room, placement, surfaces... 

..other places to fine tune your amps if your 300B tubes tested out ok.  Once you reveal other details, others may chime in, good luck  :) 

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?
.........................................

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.
.......................................

And, I was using 6550’s, liked them, just for fun tried KT88’s. I prefer the KT88’s slightly.

Looking to change my stock tubes in my Primaluna Dialogue Premium 
integrated amp, so many  to chose input would be appreciated
Power 
Per amp  

There is a big difference in terms of sound profile when you get into better 300Bs.  As someone who imports 300B amps, a big question is what tubes to go with.  
At a modest price, the Emission Labs 300B is a huge step forward from the Russian mass produced tubes and is what I ship standard with out Diavolo amps.  These are $675.  

KR tubes offer a bit...more.  More detail, slightly better linearity.  Tighter bass but are a bit more expensive.  The base 300B is $1100, the balloon which has a larger envelope is $1300.
Sophia which are largely TJ Fullmusic are wonderful tubes. Last I checked, they have the Blue glass classics in-stock.  Those are best with the matching 274B.  What rectifier does that amp use?  They are out of most of their other 300B tubes but they said the have a few Royal Princesses in stock.  The blue glass are $1000 with the better warranty.
I sell these three brands at ArtAudio.us
Acme PSVane has a great line of products and only in the US, only sell direct.  Lots of threads covering off on different variants to dig into.  One note.  Anyone claiming they are selling new PSVane/Acme tubes better be reselling a pair they bought or their is a chance they are counterfeit.  Evidently this is a big issue on eBay for them.  Go to their site to buy direct.  

Takasuki’s are amazing tubes but a bit pricey.  Made in Japan, these are stock tubes on the Luxman 300B and are one of the best current runs.  The only issue with these is they tend to perform best at a lower bias.  They aren’t as sensitive as the EML Mesh which max out at 28w as opposed to the standard 40w but operate best running a little lower. These typically are priced at $2K a pair from what I have seen.  

I have an e-mail out to them to get a price and terms for importation.  There are a few current dealers. 
NOS 300Bs tend to be pricey as you know. I would be cautious about who I bought from.  

Hello Emil!  If your amp uses a 12AX7 input tube, change it to a New Old Stock Telefunken 12AX7.  I have a 300B tube amp, too and the new input tube made a remarkable. The distributor of my amp (Ele-Kit TU-8600B) says the other tubes don't make enough difference to bother with. At $200 or so, it's not expensive for the big improvement it makes. Keep Smiling.

Thank you all for your input. Much appreciated and something to think about.
@atmasphere 
all tubes have been purchased with the last 2 years with moderate use. I think they should be fine
@decooney 
Upgraditis. Too much free time on my hands. I can easily live with what I have now but isn't this the nature of the hobby?
My system:
Audio Mirror Tubadour III SE
LTA Microzotl Preamp
Quicksilver 300B amps 
Volti Audio modified Khorns
Full Loom Triode Wire Labs cables
@boomerbillone  
Input tubes are 12FQ7 . Recently replaced and purchased from Vintage Tube Services
Rectifier tubes are genelax Gold Lion 5AR4
@Verdantaudio
The Emmision Labs 300B looks like a tube I would be comfortable with considering it's price.

all tubes have been purchased with the last 2 years with moderate use. I think they should be fine

@emil  The age of the tubes and how much use they've had has nothing to do with whether they are good or not. I've bought plenty of bad tubes that were brand new!

@emil
decooney, 
Upgraditis. Too much free time on my hands. I can easily live with what I have now but isn't this the nature of the hobby?

Sure, it can be. Or, until you achieve the title of your post and what is most engaging for you by the way of "better amp" -and- "better tubes" :)