So I too went ahead and had my 8+ year old SLP05 ugraded. Got it back yesterday along with a set of (6) RCA tubes from Cary. Tubes were matched on selection and then adjusted prior to initial burn-in check at Cary Audio to optimize gain match.
I am currently running it in using standard fuses and my emergency backup jj rectifier rather than my NOS Mullard. Also have my Golden Ear sevens hooked up during this run in rather than my Harbeth SHL5+ speakers. So the sound may not be optimized for my tastes yet. FYI, my system chain is a music-dedicated MBPro running JRiver MC21 > ISO Regen with LPS-1 power supply > PS Audio DS DAC > SLP-05 > Pass XA-30.5 amp. Running fully balanced from DAC to amp.
Of course I have l Listened a while to get a quick first impression of the sound. And I must say that I am impressed and extremely pleased with what I am hearing. Even with a few second string components in place the sound is much better than it was previously with my regular lineup. I would echo the description by Iwin. Immediately obvious was the lack of speaker hiss. Much more quiet than before even though I then had in a new set of Sophia Electric 6SN7s which I have found to be the quietest tubes of all the many that I have tried. Of course a little of the other sound attributes I'm hearing may be due to the new set of RCA tubes, but considering that I have tried eight other sets of NOS tubes along with four sets of new production ones, I think I have a reasonable feel for what the preamp upgrade brings to the party.
Therefore, in no particular order, these are the "improvements" I hear over the original SLP05:
1. Deeper, tighter and more impactfull bottom end.
2. Soundstage is somewhat deeper and wider with great image specificity and stability.
3. Tone density improved while maintaining transparency of individual instruments.
4. Very smooth, organic sound yet with excellent dynamics.
5. And most pleasingly noted by me is the sound wave impact and decay which is now much closer to that heard in a live performance. That is to say the notes now start and stop faster which is so much more realistic to me. Not sure what is the reason for this effect. Maybe added capacitance, type of resistors used or changes in power supply. All I know is that I really like what hearing. Percussion whacks are now really exciting.
Hope to hear a bit more improvement over time and with my normal configuration. I will look forward to hearing others impressions.
John

