Amp Input Impedance Differences

[mars_bringer_of_war]

I got a hold of a Peavey VTM head which I love, but...
All of my boost/od/fuzz pedals, both DIY and commercial don't boost/distort nearly as much through this amp as they do with others I've owned. A couple of clean boosters (Fetzer and Minibooster) only amount to barely more than unity when either pedal is cranked.
Would the amp's input impedance make a difference, and if so, how can I compensate?

*Edit*
Ok, after I thought about the 1st part of my question, I know the impedance makes a difference. 

[brett]

Hi
the VTM-60 has separate low and high impedance inputs, right?
The LOW is for pre-amp outputs, while the HIGH is for guitar/pedals/etc.
Do you have a schematic to confirm this?
cheers

[brett]

Oh yeah...
if there's only one set of inputs, it might well be low impedance.  A buffer can convert that easy enough.  My first choice would be a JFET buffer, self biased, using around a 20V supply (whatever is available in the amp) or 18V (two batteries) if no suitable voltage is available.  This will give plenty of headroom in the buffer and pass a large signal to the next valve stage.  A BJT buffer would be ok, too.

[mars_bringer_of_war]

Yes, the VTM is one-channel, separate high and low imp inputs.
So the buffer would be at the end of the chain, last before the amp?