Dear friends I am interested to upgrade my caps in the tube preamp from Audio theta tin foil 3uf200VDC to Miflex1uf 240VDC copper foil what are your thoughts will reducing the value to 1uf affect the bass and frequency response or should I go with a higher value than 3uf  
I like Vcap Odam much more than Miflex . Just pulled out Miflex and replaced with Odam in my amp. Much better after break in. Everything improved from bass to highs.  More depth, more inner detail… get it. 
Miflex are forward sounding and not as smooth and detailed as Odams. Odams are even better sounding than Jupiter copper in my experience, but I shared this already. 
I understand what you say, but then 18pcs of Odam 0.33uf 600VDC is some big money and miflex is less than half the price of Odam

I would advise to not change all your caps at once. Change maybe one or two pairs at a time and sit back and listen. If your sound goes in the wrong direction, you can alter your path. If you feel the sound is on the right path, you go ahead.

Each time you will hear an improvement, so you get to enjoy the improvement every time. 

Another benefit would be distributing your costs over time, so you can buy really good quality capacitors. 

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.
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