McIntosh MC275 VI preamp tubes

I can tell that one or more of the preamp tubes (12ax7 and 12at7) are starting to get weak...the music is starting to sound a bit muddled and not quite as clear as it used to be.  I need to replace one or some or all.  I am looking for suggestions for tubes.  I love NOS RCA tubes, but they are getting harder to find and a bit pricier than they used to be.  Has anyone tried the Golden Lion, and/or the newer black diamond telefunken?  I think I tried Tung-Sol, but didn’t care for them much.  I’m open to suggestions, I’d like to keep the price roughly around $50/tube.  
I have golden lion KT88 that still seem to be going strong.  My system is:

CD Player : Marantz SA8005
Preamp:  Krell KSP 7B
Streamer:  Cambridge Audio CXN (V2)
Speakers:  B&W683

Any suggestions would be helpful.  Thanks !!!