I don't know about the 2300 but I have the C220 and have replaced the stock tubes with some NOS RCA's and some old Telefunkens. In both cases they improved the sound quite a bit.
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I tried the Teles as mentioned above and found them just ok nothing special IMO, you would figure with all the hype and high prices the Teles bring that they would be the go to tube......I also used early RCA black plates and prefered them over the Teles. I decided to give some newer production stuff a try and ended up with re-issue Tung-Sol for the preamp section and re-issue Mullards for both phono sections and so far this combo is my favorite. Something I noticed with the C2300 is that tube rolling doesn't drastically change the sonics of this preamp it's more subtle compared to other tube preamps. Sort of nice not having to spend $$$ on over priced NOS tubes to get the best from the C2300 IMO.
Tele's work the best in my system and I have owned and still own 10 other brands. I keep going back to the Tele's. They just do it all right with this preamp.
This preamp really shows the character of each tube. Then of course your speakers, room and other things like cable will make a big impact also.
I have a pair of vintage RCAs on the way...12ax7a. I am using JAN Phillips Black Sable 5751s now after a week with Tung Sol 12ax7a tubes. I like the Tungs better.
Cant wait for the vintage (early 1960's) RCAs to arrive!
All of the above improved sound greatly over stock Mac 12ax7a.
BTW: the amps are 501s.
I absolutely hated the stock line stage McIntosh tubes in My C220. I purchased some Black Sable Mullard Blackburn UK made ECC83/12AX7's from a fellow Audiogoner and was still unhappy. I had scored some 1968 Raytheon 5751's windmill getter tubes a few years back for my Dynaco Pas 4 (Panor Corp) and decided to try them in the C220. WOW! I've rolled some 12AX7 tele's and 50's 5751 GE triple mica blacks and I'm thinking the Raytheons are the ONES. Very interested in trying some vintage Tung Sol 5751's. The more I hear of these 5751 tubes the more I seem to prefer them over 12AX7. They just seem cleaner and tighten everything up. I'll continue trying the 12AX7'S from my cache with serious listening attention, but for now those Raytheons really do kick some ass in a Mcintosh C220!!