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  
Some general recommendations:

- Do NOT change the capacitance value of your capacitor. The design calculations are based on its existing value.

- DO change it as long as you are INCREASING its voltage rating. Never any harm in increasing its voltage rating.

- Do NOT *EVER* replace a bi-polar capacitor with a polar one, otherwise it may blow up.
You can change the value of an output coupling cap and do so safely and do so to better effect. But, as a general principal you want to stay near tolerances as designed.
Sometimes people violate rule no 2 above because they want to better match impedance between amp and preamp. Manufacturers usually try to pick a level that will match with a larger number of combinations yet you can customize this so the gear mates better. There are good calculators out there for this purpose.
I hardly ever check this forum any more but I did today and I read this entire thread.  And it reminded me why I don’t read threads here.  Same people arguing about the same topics, over and over.  Don’t you get tired of this?  You guys need to move on!
As far as what brand capacitors to purchase, the two choices for us were V-Caps and Audio Note.  In comparing Milflex copper to V-Cap Oils, we preferred the V-Caps.  The ODAMs are an excellent option and the Copper V-Caps are excellent but expensive.

Happy Listening.