Which NOS to use on Audio Research pre amp?

Gold Aero if you could get them.

I currently own an LS5 MKIII and I have tried the original Sovtecs and Telefunken E88CC (6922).

In my set up, I found that the Sovtecs put the soundstage at the front of the speakers and I was surprised that the instruments were very sharp and focused. I was expecting less than this, to be honest.

The Telefunkens, on the other hand, added more air and depth. This put the soundstage between my back wall and the speaker.

Note also that I also using an ARC VT130se amp that was modified by Great Northern Sound, using GE 6550 power tubes and Siemens 7308 input tubes along with Coincident Super Eclipse speakers.

Tube rolling, although frustrating at times, makes a huge difference, in my opinion.

Steve

Thank you everyone for your opinion. I will give the NOS tubes a try and swap out the Sovtek's which I think is making the sound bright, everything else seems to be okay, but I with the right NOS tubes it will improve.

Use 4 Siemens in my LS-15. With infini cap upgrade and try DIY power cables. Now I am comparing this one to BAT VK30, which is pricer using 6 Jan Philip tubes now. I prefer LS-15's middle frequency over BAT. BAT has better bass control than LS-15. Siemens improve the high and low end, IMO.
Now, plan to keep LS-15 and sell BAT (both are very nice preamp's). I did not have 6 Siemens to put in BAT for a more complete comparison. But I am pretty happy the way LS-15's sound now and will keep it as now for some time.