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.