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.
I use Mullard CV4024 in the phono section of my VTL TL5.5 and they do exactly what you are looking for. They are anything but flat and and have solid bass.

I would also suggest Sylvania Gold Brand 6201s. I use them in my ST 150 amp and I like them even better than the Mullards. To my ears they are a little fuller in the midrange and have a touch more weight in the bass than the Mullards. But they are not that much better that I feel like I want to buy another pair and replace the Mullards I already have.

Sorry I can't make any informed recommendations for 12AU7s.

I used a matched quad of the Mullard CV4003's, (from Upscale Audio) in my Belles preamp and was totally blown away. I had tried numerous others and found these to be excellent.
The VTL 2.5, in stock form, is anything but flat or lacking in dynamics. I would suggest getting a fresh set of stock tubes to use as a point of reference. They should be relatively inexpensive as they are current manufacture tubes.
Try Andy at Vintage Tube Services. Not cheap, but the tubes I bought from him were exceptional (Telefunken 12AU7).
My McIntosh C500 preamp has matched quad of Black Sable - Mullard ECC83/12AX7. Check out Tube Depot Preamp Tubes
Thank you all for your response...I will contact Kevin Upscale Audio and Andy Vintage Tube Services.

More suggestions are welcome.
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.
Thank you Lapierre. I will contact Doug.
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.