That's great news.
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According to the user manual,
your preamp takes 6922 tubes in the linestage. 12AX7 and 6922 in the phonostage.
What are you using for a source?
In any event, take the cover off the preamp and pull one of the tubes from its socket. You can then see the brand of tube being used. You'll need to replace the 3 linestage tubes. (If you're using a turntable as a source, you should also replace the phonostage tubes).
To give you an idea of various brands and prices check out the following link...
New production tubes are made in Russia and are very affordable. Vintage tubes are higher quality from the good old days before transistors.
But first identify the current tubes. If you liked the way your preamp sounded, you can replace with the same brand.
I owned a 300, very good amp.
Did this muddy sound appear gradually? If so, this is how dying tubes behave.
The mono type of sound could also be tubes. The bias may have changed due to lower voltage to the tubes, IOW, tubes are weak.
Music will sound less dynamic, less detailed, soundstage will diminish when tubes are dying. Are you experiencing this?