Upgrade capacitors in tube amplifier?

I have upgraded crossovers with very good results.  I now have a "very good" IMO chinese tube amplifier that can probably be improved with a couple of capacitor upgrades.  

I don't have a schematic, but I have a picture and I expect some of you knowledgeable folks can point me to/confirm the ones to change.  My thought is to change the two large silver ones first and then maybe the four smaller silver ones (on the circuit boards). 

Would probably replace with something like Jantzen Superior Z-Cap or comparable and go with a higher voltage if space permits.  



Thank you all for your input. 

I will do as recommended by many of you: roll tubes first and then consider the caps.  I took the bottom off the amp and found that the caps mentioned by most are two large 0.33UFD 5% 630VDC and four 0.022UF 5% 630VDC.  I realize you can't tell much by looks but they did look pretty good.  They are oval.

I have some ECC82 (Tung-Sol black glass) and 5751 (GE black plate) on the way and now I'm trying to decide on the EL34/6CA7.

Thanks again.
For your coupling capacitors (particularly the 0.33mfd), and importantly, if you physically have the room, consider either Jupiter Copper Foil Paper & Wax or Miflex KPCU-01. The Jupiters are a bit more expensive but you won’t go wrong with either.

I’ll second (or third) the suggestion for Takman REX Carbon Film resistors. Very musical.

While you’re at it, consider the output transformers. Look into Lundahl.

And look at the power supply rectifiers. I don’t see a tube, so it must be silicon. If it has Schottky rectifiers, you could consider replacing with (third generation) silicon carbide (SiC) rectifiers.

And has been pointed out above, once you start down this road, you’ll probably end up spending as much on component upgrades as the initial cost of the amplifier. Be sure your comfortable with that.
RAM tubes, best computer matching on planet

thanks to the genius of Roger Modjeski ( RIP )