Min -
It's always good to keep the AC away from the inputs
as far as possible when doing a layout. Since you have
a small box, everything is pretty close. Aluminum isn't
much of a shield against magnetic fields. You notice in
the original Shure picture, the transformer was on the
outside of a steel chassis, and the tubes had shields.
You could probably easily rework your box to move the
transformers outside, then move the diode/cap strip
over behind the transformers, instead of near the
input connector. I might also move the outputs to
the far set of jacks, proximity of input to output is too
close for me. Try Ed's "unplug and listen" suggestion too.
Preamps are probably the trickiest ... and I do mean
"trickiest" since the ubiquitous Lafayette stereo preamp
which sold for about $ 12.95 when I was a kid was
a gutless wonder, but of course they are bringing $$$
on eBay these days.