Suggestions for 12AU7 & 12AT7 Tubes - VTL 2.5


I have a VTL 2.5 Pre-amplifier. It has 2 x JJ 12AU7 and 2 x ARS 12AT7 tubes. The sound is flat, without a good soundstage / imaging and lacks bass and dynamics.

I would like to know changing which tubes make the most impact in VTL 2.5

(1) Change the 12AU7 / ECC82 tubes
(2) Change the 12AT7 tubes
(3) Change both 12AU7 and 12AT7 tubes

After doing some research on the web, I have come across the following tubes (open to other tubes and to your suggestions) :

12AU7 - Mullard CV4003; Mullard M8136 (NOS); Siemens E82CC
12AT7 - Mullard CV4024; Mullard 6201; Valvo 6201

I am looking for low noise tubes with extended highs (no HF rolloff) that can address the above mentioned issues and improve the sound of my pre-amplifier.
bssk
Bssk you may find that these tube vendors may not have all the tubes you want. Good you have options. Kevin is great resource as well as Andy.

Also try Doug's Tubes

Doug knows his tubes.
I got the Mullard M8136 and the Mullard 6201. I hear a difference in the midrange especiall in vocals. There is a reduction in the midrange harshness. I feel that the amplifier has a greater sense of ease, and I can listen to music much longer hours without fatigue.

Thank you all for your suggestions.