preamp gain problem.

I would first suspect that someone may have substituted the original tubes with "low noise" tubes, ie lower gain. There may also be jumpers within the pre that change the gain. Check with VTL, hopefully you can talk to a tech and not Luke Manley, the rudest, most arrogant person I've ever dealt with. What an ass.

Belles amps are designed for preamps that have high gain to drive them. One only needs to look at the high gain (at least 20db) of the Belles preamps. However some people do prefer a preamp of lower gain where normal listening levels are around 12 oclock. This lowers signal to noise ratio and you never have to worry about clipping or over-driving the amp. I personally prefer this scenario because it sounds better to my ears.

Juancgenao, how much gain does the VTL pre have?

Get an old ARC tube pre. I own an ARC SP6B. This set up and circuitry seems to be a common theme for many of their preamps. Mine has gain from hell and tubes.
Even though I am a devoted tubophile most old stock tubes make some noise. It takes very careful selection from a batch of new or current production tubes to find the few quiet ones. BTW I tried the switch to lower Mu tubes and it had almost no effect. If it is a high current pass through tube like a 12AU7 forget it your woofers will wobble like a bowl of jelly.
The phono is amongst the legends. The later ARCs seem to switched to the 6922 for a quieter outcome but they aren't necessarily quiet either, believe me.