I hear ya'
If the tubes are old than maybe it's time for a change, you'll need them sooner or later. Remember, there are many reasons why an amp can squeal or feedback. A bad tube is one of them. So are the electrical appliances around you, they help with the excess noise and I believe this is called interference.
Which takes us to the Power Conditioner. A power conditioner regulates and cleans the the current being drawn. Basically your mark IV is drawing electricity from the outlet but the outlet may not be sending a constant amount of electricity that the mark IV needs for it to run at it's prime. A power conditioner essentially means your mark IV will be happier and less noisy. Now, pretty much everyone uses Furman Power conditioners because they have a reputation. You don't need anything special, I recommend the PL-8.
For example, in my neighborhood, we have horrible electricity. The houses and lines were built 80+ years ago. We still have 2 prong outlets. I had to change a few in the house and stuff. Anyways, after I get my mark IV cabinet and guitar back, I'll be getting a new set of tubes from Doug's as well.
Best of luck