Tracking down a hum.

Today when I powered up my system, I noticed a hum from my tube pre amplifier, a Rogue Audio Model 99 (line stage only). It is worse when first powering up. It is absent through my headphones, (using an external headphone amp and the record button on the pre-amplifier)  Listening to speakers the noise is present but diminishes and eventually goes away as the system warms up.
I'm using the Rogue to send a signal to a Musical Fidelity X-P200.
I don't think the issue is the tubes as I'm using Tung-Sol 6sn7 tubes with less than 300 hours on them.
I bought this Rogue used and had to take it back to get it fixed as one channel kept cutting out because of a bad solder in the out put jack. Maybe something else failed?


@noroamnce, kudos. I'll know more about which caps were replaced when I see the invoice on delivery.
@thecarpathian. The explanation given to me is they were unable to get schematics with values and were reluctant to work on it as such. Rogue would not provide them either.


Okay I got the amp back today. I was hoping for an itemized list of parts replaced. I didn't get one. It looks like the replaced all the capacitors in the board nearest the transformer, and one resistor was replaced. I'm listening now and so far so good.

Well after listening for a couple of hours out of nowhere the noise is back.Evidently the folks at Rogue missed something. This time the noise doesn't dissipate. The noise pulses. get louder then softer louder then softer. I have no idea what the cause is now. I'm thinking a faulty rectifier.
This is frustrating and starting to get expensive.