ideal tube for VTL mono 100 amp. monoblock

Replace the Mullards with either NOS Siemens or Telefunken 12AU7's. British tubes typically have a softer, warmer coloration than German(way more transparent and extended, without any in-your-face glare or stridency). Amperex made some nice tubes that fill the middle ground.

Another thing: If your power amp has user adjustable bias- Check that and confirm that it is within spec. Low bias will result in an un-dynamic, recessed sound from an amp.