Crack/Speedball - hearing some distortion in the R channel

[Mr. Electric Wizard]

I tried moving the power cable over being plugged directly into the wall (it was in a power strip before).  Still have some static.
I have REW installed on the PC and it has a section to play sign waves.  I played sign waves from 1,000Hz down to ~45Hz to see if I could hear any static while just playing tones.
I never heard any static, but when I had the tones playing ~300Hz and pulled the volumn pot up from 9o'clock up to 12o'clock I could hear the right channel tonality changing (getting more bright maybe).  There does seem to be some issue internally to the Crack.  Just not sure what it is. 
I've re-flowed all of the joints, and made sure there was solder on all the joints.
I had several days of listening to the Crack without the Speedball installed and I never heard any distortion.

We are talking very faint distortion.  But it's still there, you know.
The distortion is only happening when the volume is louder.
It's not there when the volume is lower.

This is my first tube amp, so is it possible that distortion is just 'a thing' with tube amps at higher listening levels?
My headphones are 300 ohm (Sennheiser HD6XX)

[Paul Birkeland]

Have you also re-flowed the joints on the Speedball boards themselves?

The Speedball lowers distortion and increases power available to the headphones, so there shouldn't be an issue like this appearing where there wasn't one before.

[Mr. Electric Wizard]

Have you also re-flowed the joints on the Speedball boards themselves?

The Speedball lowers distortion and increases power available to the headphones, so there shouldn't be an issue like this appearing where there wasn't one before.

Yep.  I will monitor for the time being and if it persists, I'll do it again.

[Mr. Electric Wizard]

Been playing at normal listening levels and it's sounded awesome all afternoon.   
I have it sandwiched between a computer monitor and a shielded bookshelf speaker.
If it comes back, could it be interference possibly?

[Paul Birkeland]

Interference manifests as noise generally, not distortion.

[Mr. Electric Wizard]

Okay, I think I've got a more accurate idea of what's going on.
If the volume is turned up past 9 o'clock there is some static in the upper frequencies.
Really that's more than enough volume for me, so I guess it's a non-issue.

[Mr. Electric Wizard]

As per my other thread, I am going to try to get a new Speedball wire kit, then remove the boards and reflow everything (including the Speedball boards), then re-install with new wire.