Strange distortion 5E3 help

[paddy]

Hi guys,

I have just finished building a Marsh Amps 5E3 for a friend.It sounds great
but has a strange noise/distortion when playing the bottom E and A string together.Both strings played by themselves sound fine but open E and A
fretted at various positions have this buzz/warble riding along with the notes.
I suspect this may be intermodulation distortion but I'm not sure.
I have tried the following fixes but all to no avail.

1. Changed all tubes(haven't tried NOS 12AY7,just new Sovteks)
2. Reduced 1st stage coupling caps to .01uF
3. Changed O/P grid coupling caps to .01uF
4. Replaced PSU filter caps
5. Increased " " " to 100uF plates,40uF screens
6. Tried several grounding schemes including the one recommended by
Marsh Amps and Hoffman but haven't tried the brass plate.
7. Isolated speaker jacks
8. De-coupled all preamp stages with 22K and 22uF
9. Increased O/P grid stopper resistors to 47k
10. Decreased O/P grid leak resistors to 100K
11. Tried a different speaker cab
12. Fitted 1K screen resistors
13. Separated mains ground from circuit ground

The 1st kit I built was a Mission Amps and I don't recall this noise.
All voltages are very close to those on the Mission schematic.

I have attached a clip in the hope that someone can tell me what I'm
hearing(is this perhaps normal?) The clip is a bit muddy as I have the guitar tone pot all the way down and neck p/u selected to make it easier to
capture the recording.

Sorry for the long post.

Any help/suggestions will be greatly appreciated.

Best Regards,
Paddy