Preamp Output Capacitor: Mundorf Supreme vs. Supreme Silver Oil

Those Duelund tinned copper caps are so special. If you can afford those, they will easily surpass the Mundorf caps in every sonic area. I think you suspect that is true.  They are pricy! 

Yes, actually the original reason I started modifying the capacitors was because the miflexs were right on the edge of being too small for my amplifier's input impedance.

Be careful- just increasing capacitor values can get you into trouble. The output section probably the least, but in many cases, the designer is juggling the inductance that a larger capacitor has against the capacitance- and has arrived at a value that has the best compromise between bass bandwidth and coloration. Bass bandwidth good, coloration bad :)

(This is why we have direct coupled outputs on our preamps- this problem is avoided.)
There is a timing constant involved (which sounds like you might already know) the formula is F = 1/R x C x 2Pi
Its a bit inconvenient as stated, but if you replace 1,000,000 for the 1, then R will be in ohms, C will be in microfarads and F will be in Hz. Ideally you want the -3dB point (which is what this formula gives you) to be no more than 2Hz- allowing the preamp to then have no phase shift at 20Hz which will give you neutral bass. R in this case is the input impedance of the amplifier.

I'm in the same boat with Don Sachs II and 2 outputs. One for 100 kOhms (tube amplifier) and one for 20 Kohms (Accuphase). In the first one is a Miflex 0.47 uF and in the second one AudioCap Theta 3 uF bypassed by russian teflon 0.02 uF. Studying an upgrade for the second out but capacitors of 3 uF are very large. Maybe a Mundorf will fit. Interested in your topic. From Brazil.

@atmasphere Thanks Ralph, I've just been cheating by using the calculator at vh audio (https://www.v-cap.com/coupling-capacitor-calculator.php). Fortunately because the pre is basically an advanced kit, I do have some ideas of what range is normal for use. 

I also just leaned that I can change the resistor on my amplifier (diy first watt f4s and sit3s) to a any value within a decent range. Moved both up to a 200k input impedance to allow for the use of smaller caps.

@grannyring and yes, definitely putting some Duelund's in. Unfortunately I didn't know my amp's input impedance was modifiable... I actually have to re-buy the exact same caps I sold you. It's been fun learning about building/modifying equipment. Still mostly playing Legos, but learning is fun.