Capacitor

2 uF is almost perfect.  It's got low reactance and most boutique caps aren't too physically large at this size.

Of course, if you have lots of space and lots of money go with 4uF. :)

@grannyring 

With tube amps you are good with output caps from .47uf to 2.2uf.  If you want a more holographic image I suggest two cap brands.  
- VH Audio Odams
- Duelund CAST CuSn - tinned copper version

Given the wording of your post, I was wondering if the caps you mention had a drop off in performance with values higher 2.2uF? I was looking at using 3.3uF Odams. 

Bpoletti, I am sorry but in this  case you could not be more incorrect.  Honestly. 
Pauly, no worries as the 3.3uf will sound great. Just wonderful. Order matched sets within 2% from the supplier.  Also, be sure to make note of the outer foil end and directionality of these caps. 

Then why are you trying to correct an obvious design error rather than looking at a more carefully designed instrument?  

The Miflex caps offer great performance for the price. They are modeled on the Duelund caps which are insanely expensive.

Obligatto caps also offer great performance for the price. 
Used both brands when recapping my tube amp and have also used in other applications.

Miflex has served me very well.

As far as changing values, that is fully dependent on the circuit design…