I put the RI's in every Marshall I have/had except my 1974X. (JVM410H, Vintage Modern 2266, HAZE 40, DSL401, DSL100). In every case the RI'S ended up giving the amp more "Marshall" tone with the sacrifice being a minimal drop in gain, which in my opinion was well worth it because the tone was just that much better.
I have a Mullard reissue that I bought a while back as a spare. It remains a spare, because it has consistently sounded and responded like thin wet cardboard in every position in every amp I've tried it in except for FX Loop or Reverb driver. The only new preamp tubes I care anything for are the Shuguang 9th Gen and the JJ ECC83S.
I don't like the RI Mullard 12AX7's.. but to be honest they don't sound like the real ones IMO. The real ones are pretty nice sounding in low-mid gain amps.. Just make sure they're not microphonic though.
The Mullard RIs have no relation with real Mullards....so in fact they are not RI they are just new tubes. It's just marketing.
Compare side by side with a real Mullard.
By the way if you have NOS tubes, you should keep them for your mic preamps. Soon or late we will have nothing to plug into our mic preamps.
I've got about 8 of the Ei Yugo ECC83's from the 70's and 80's, which are smooth plate Telefunken copies, and they sound GREAT in Marshalls. Little hard to come by, though. I think Doug Preston has them for about $30 a piece.