Better amp or better tubes?

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!