Capacitor

@jasbirnandra

I cannot speak for your pre, but I’ll share what I have on mine.

I have 4 x 3uF coupling caps.

1 pair (the pair towards the back of the chassis) are output caps. I confirmed by measuring the resistance between the output RCA and the cap’s red lead and measuring between the 2nd pair of 6SN7 (the pair towards the back of the chassis) plates and the cap white lead. 100% they are output caps. This pair I may consider going smaller to get the best quality cap.

The other pair are interstage caps. I confirmed by measuring the resistances of that caps red lead with the grid on the second pair of 6SN7s and the cap’s white lead with the plate on the first pair of 6SN7s (the pair towards the front of the chassis). 100%, they are interstage caps. This pair I will stick to 3uF.

My pre also also has 2 pairs of 0.01uF caps - one per 6SN7 tube. In both cases the red lead is connected to the grid of triode 1 and the white lead to the plate of triode 2. IOW the two triodes in each 6SN7 has a coupling cap. I’m going to do these first as this is much cheaper - I just have ordered some VCaps a few minutes ago.

Note, this is the C800 MK2. How your pre differs I don’t know. I’m simply sharing what I have found.

Why do you think changing caps will give you the desired results?  It's the entire preamp design, not one circuit component, that makes the difference.

You'd be better off selling the unit and searching for a different one that will give you the performance you want.  If you change one cap in the circuit, the manufacturer may decline to make repairs to such a modified unit.

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.