I now have a Monarchy NM24 DAC/Linestage which uses 4 6DJ8's. I changed out the stock Phillips and substituted some Amperex Bugle boys and have experienced hissing,dog whistles. I had a matched set of 8 and changed them 1 by 1.
I was able to eliminate the hissing but the D whistle persisted in one channel. I continued to substitute different tubes and it finally disapeared.
I must say low level noise can have many sources. Here are some things to check.
Remove all sources of RF emitting devices from your listening room. This includes portable phones,cell phones,remotes ect.
Check that all interconects are seperated from each other as best you can. Power cables should not be in close contact the interconnects or speaker cables.
If possible think about having direct circuts for your sound system. I have 4 20 amp Porter ports which are Hubbel 8300H hospital grade plugs. This cost me about 800 dollars for the wiring and 150 for the ports well worth it in my book.
Checking all these things and your issues are'nt resolved. I can only say it can be one big pain. I would talk to your manufactuer tech's and see what they think.
BTW the Monarchy sounds extremely musical with the BB's and I really like eliminating 1 pair of interconnects in my system. I am having a Accupuhase C200 upgraded to lower the noise floor. The C200 was extemely quiet but I am hoping for improvement. I will post a squib on my system when I get it complete in about 2 weeks. Good luck on your problem.