Capacitors for Audio Research Reference 5se

The V-cap is probably better than what they put in when it was built, so what are you worried about? Wait until you see how it sounds before you start worrying about replacing it.

I know that V Cap needs about 350 - 400 hours of work to fully show its advantages. Unfortunately, at the moment, after only about 50 hours of operation, they are still very far from the original capacitors, so I am looking for an alternative solution in case they do not fit into the preamplifier.

Yes it is true that V Caps take several 100 hours to break in (I have CuTF, TFTF and OIMP).  You should have built a jig and burned them in on your work bench before installing them, you could always remove them, burn them in and then re-install.  

A jig can be as simple as taking an old receiver, or similar, connecting a 4 or 8 ohm power resistor (I use 50 watt) in series with the capacitor in question, or several wired in parallel.  Turn it on and increase the 'volume' over time monitoring the temperature of the resistor and output devices/heatsinks, as a rule of thumb, if you can comfortably touch the resistor and output devices/heatsinks you should be OK.

Are you satisfied with V Cap capacitors and what do you think are their most important advantages?