Tube Amp, Preamp upgrades -- what did you do that improved your units?

Tubes are, i had thought, already a given. I’m thinking of alterations to the machine.

Also, learned this: the Orange Drop caps are 90 cents (.90) a piece at Tube Depot. 
https://tubedepot.com/products/orange-drop-715-series-022f-600v?utm_source=google&utm_medium=cpc&adpos=&scid=scplpCP-OD715-022-600V&sc_intid=CP-OD715-022-600V&gclid=EAIaIQobChMIsIa6_8yH9AIVeQytBh0nbQ4EEAQYBCABEgICr_D_BwE

some of us remember swapping in Orange drops into a Hafler…because they were an…upgrade… it’s all relative….

I would do what I said above with your preamp.  Replace the Alps Blue pot with the same value TKD. The potentiometer has a sticker on the back that tells you if it’s 25k, 50k, 100k etc…. Nice step up in resolution and realism. More than replacing a cap. This can be found at Parts Connextion or Hifi Collective. I think the TKD pot costs just under $100 shipped. 
Replace those white caps labeled QS behind the tubes, 2 of them, with Vcap Odam 2.2 uf.  600v. If too pricey for you use the Audience XO caps. The Odams are wonderful in every way and a nice step above the Audience. The Odam caps will cost around $100 each.  The Audience caps about 1/4 that.  
You could replace the 4 - 47uf 500v filter caps with Mundorf M-Lytic HV 500V 2-Lead Single Section Axial cap from Parts Connextion.  If the amp is 10 years or older, then consider this upgrade.  They cost some $11 each. 

@grannyring Thank you. This is all for the preamp. One thing -- the amp is not that old -- it was probably built in 2020 or 2019. Would you still replace those 4 - 47uf 500v filter caps? It sounds like you'd only do it if it was an older model. Thoughts?

Also, did I miss your thoughts on the mono block amps? 

@tomic601 I suppose it’s all relative. My amp is a 2020 model so I’m not sure how those orange drops stack up, relatively, *today.*

Your amplifier has 4 coupling caps that can be improved on.  The two orange ones are .22uf and are Orange Drop brand. Replace with Vcap Odams if you can afford them. Audience Auricap XO if you need to spend less.  Both will fit.  
It also has 2 white QS branded caps that are an odd value .27 uf.  Most all caps come in .22uf. Don’t just replace with .22uf.  You want to match the .27uf value. Both Audience XO and Sonicap Gen 1 from Sonic Craft have .22uf and .047uf value caps that can be combined in parallel for the .27uf value. They are both not expensive. 
The bridge rectifier is soldered directly to that large blue electrolytic filtering cap mounted on top of the chassis. It can be upgraded to fast recovery type (Fred) if not already. Not sure what is in there now. 
Other stuff, but not sure you want or need to go that far.