Tube substitution in VTL amps

I'm interested to find out a bit about tube substitution. I purchased a set of VTL amps a couple of years ago (2.5 & 2xST85) shipped over to the UK. These are my first tube amps and I'm basically delighted but occasionally miss the slam / bite of my previous solid state kit. I gather the EL34 is fairly well known for a softer sound and wondered if it's possible to get more bite by putting different tubes in the pre or power amps? Would this have any downsides as I assume the amps are designed for EL34s? Would changing tubes unbalance the sound at all? Any pointers would be much appreciated.
Firstly, you can't place anything instead of EL34's in your amp. These tubes are well known for their natural mid-range and voice reproduction. Mullards have the strongest bass among all EL34's but may be very costly. I use in my MB100 EI34 Yugos and I can call it the best budget tube to any EH, Svetlana or Tesla. Stronger bass than Svetlanas and EH together, tonal balance and great highs and midrange. Had a bad luck with EH tubes going bad too fast probably due to the different plate voltage ratings than stock Svetlanas. Additional modifications are available for the amp to use different tubes such as KT90, KT88, 6550 that have much more dynamic capabilities and bring out much more power. It would be too costly and meaningless to send an amp back to US if you don't have any VTL technician in UK, but I believe that these modifications can be done by any tube-literate technician near you or near UK otherwise.
Some amps that use EL 34's will also accept 6L6's. CHECK WITH THE MANUFACTURER FOR COMPATIBILITY!