Mac 402 & Audio Research LS25MKl ?

Make sure your system isn't "new to you" before you go changing things. It will make your future changes a lot more sensible.

I am using the 6H23 NOS Type 2 from Upscale Audio in my LS25 MKI with excellent results.

Chiming in a little late here, but, I second Gmorris as I'm running the same tubes in my LS25 MKI, much better than the stock Sovteks. Other than the type 2 EH tubes from upscale, I haven't rolled any others. I don't know anything about the MAC 402, but if it's a truly balanced amp and your not using XLR cables between pre and amp, you should try it. Switching my system from SE to XLR, specifically between pre and my amps (JC1's) catapulted my system to another level entirely.

only thing i can suggest send your ls25 to ARC have it upgraded to mkII ..you will notice an improvement ,your mac 402 is a top notch amp ..

Seems to me that you have a heck of a good system. you didn,t described your room or your room treatment. But having that nice pieces of gear and not having attention as to the best in room acoustics would be a waste. I would have an acoustical expert or company like rives take a look at what can be done. BTW; While I have heard at a friends system the ls25 mk2 and its a very good sounding pre its disadvantage its that you are stock with the 6h30 tubes and no 6922 tube rolling if you go into that direction. Good luck on your descicion.