Capacitor

18 pieces! How can that be? Well yes that is quite some money. Please remind me of the amp model we are talking about here and the positions you are using them. I assume not as bypass caps in the power supply?  Thanks.
Bill

I am using the Canary Grand reference with 8 tubes on each side when I opened the amp cover each tube had a cap underneath it 

Yes, those are coupling caps and are certainly and important ingredient to your amplifier’s sound.
Those are wonderful amplifiers with great build quality. I owned the 339 monoblock Reference amps and enjoyed them tremendously. Canary used Hovland capacitors for years. Hovland only makes the black colored Supercaps now. Are they in your amp now? Perhaps the older blue or yellow colored Hovland caps? Please let me know. Please let me know what’s in there now and I can help you decide if an upgrade is worth it and which ones are worth the time, money and effort.

The Miflex are forward sounding. If you want a more immediate and forward sound, then they will do that. I found their presentation was impairing my ability to enjoy music over longer listening sessions, Just too forced and compared to being at ease and set back a tad further.

Those amps deserve the best and will reflect the sound quality upgrade the right part will deliver. If you have the black Hovland Supercap, then I would not replace with Miflex. Not a worthwhile investment in time, dollars and effort.

The current caps are the Audio Theta ppk 0.33uf 600VDC

" I am using the Canary Grand reference with 8 tubes on each side when I opened the amp cover each tube had a cap underneath it"

You will need to do all the power tube caps at the same time. Sounds like you have 16 power tube caps? Ouch - that's a lot of caps you need to do. 

I'm not even going to ask what it will cost to replace the tubes ... 

Good luck