First off, don't start doing mods to that amp or you will get a lot more noise in the higher gain states. These amps can be noisy enough unless you hand select preamp tubes.
Have your bias checked! If the amp is over or under biased, you will get all sorts of grainy nonsense down in the range you mention.... especially if the amp is overbiased and running too cool. These amps already start to breakup very quickly in the preamp stages, as that is what they were designed to do, so it is even more critical to have a good output stage after the preamp stage. I would bet, after looking at a number of these, that with your volume set to 3-4, that there is a lot of crossover notch distortion if you scope the amp.
To give you more headroom, you need to have somebody look at your first two preamp tubes to see how much above or below 1.2mA they put out with 250 volts on their plates and a -2v bias. If they are the typical Sovtek/Fender tubes, they are probably putting out more than the spec 1.2mA. The 12AX7 has a gain of about 100, so the difference between 1.2 and 1.3 is pretty dramatic.
You need to find a nice 12AX7, perhaps a 12AX7C, that has maybe 1.0mA output. The other thing you can do here is put a JAN 5751 in the V1 position.
Some good places to get either the JAN or a bit lower powered 12AX7 are Watford Valves, Tubeworld, and KCA NOS tubes. All those folks have links on my website. They are the only folks that have the equipment to measure the qualities you need AND have the tubes you'd want if you go the NOS route.
Then, you need to have your output tubes checked. If they are in a midrange, you'd want them to be at a higher range. Something like, if they are a Groove Tubes 4-5 rating, you'd want them to be a rating of 6-7.
Last, your phase inverter, let me know what is currently in there. It is the preamp tube that is closest to the power tubes. Tell me what it says on the tube, what kind it is, and as much as you can tell me about it. This is a critical tube in this amp, especially when you run the amp Class A, as if this is not balanced on the A/B sides, there will be a lot more heat generated than is already done when the amp is in Class A mode.
When you have the bias checked, find somebody uses a scope if they also use a bias probe. Many folks use a bias probe incorrectly. They might use charts or manuals that say what the correct reading for a particular amp should be, and this is often a bit off as different amps have different B+ voltages. Folks that are more technically advanced, try to figure out their real B+ voltage bu pulling one of the output tubes and taking a reading of the plate pin. This is going in the right direction, and is easier than taking the amp apart, but also gives a false reading. When one output tube is removed, the voltage changes a LOT in many amps. In one Marshall I work on, a 50 watter, it goes from about 450v to about 415v when both tubes are in the amp.
To get the proper B+, ESPECIALLY on a Prosonic, you need to pull the chassis and take the reading off the wired side of the socket with all tubes in the amp. Then you can use a bias probe more effectively.
Regards,
Ok, I finally got around to this. The rectifier tube is a Chinese 5AR4 GZ34 (Fender on it).It has sovtek 12ax7wa's throughout with exception of the reverb tube. It came with sovtek 5881WXT 6L6 power tubes. I have more recently installed Audio Classic 6L6WGB matched set at a 10 rating. It made a bit of clarity difference but took away some warmth. What do you think my next steps should be?
Thanks again!