Line Magnetic 501-a gold? any experience?

If you are experiencing transformer hum, you might want to try a DC blocker. PS Audio used to make the Humdinger to help with this, but the best option now seems to be Van Alstine's Humdinger. $125 with a 30-day return policy. So if it doesn't solve the problem, you are only out shipping (and it is a small light device.

For toroidal transformers, I highly recommend isolating the stupid thing from the printed circuit boards and electrical components using visco material under the transformer or cork or combo. Mu metal of the annealed variety should be wrapped around the transformer at least once.

I recently auditioned the LM 518 mated to a pair of Harbeth 7es-3s, the same combination Tim Smith used in his wallofsound.ca review. It sounded absolutely horrible. Congested, bloated, dull, slow, and completely devoid of any dynamics. Everything from the lower mids down was a complete mess. The dealer was apologetic and as he said, "embarrassed" that I set a Saturday aside to drive the considerable distance to hear (and not hear) this highly regarded amp that we both agreed was unlistenable. I was really wanting to love this amp and was sorely disappointed when it wasn't even close. The dealer attributed the poor sound to what did seem to be a rather pronounced echo in his listening room, however, it sounded so poorly, I wasn't convinced that could be the primary reason and declined his generous in-home audition offer. As a consequence I've since ordered the Coincident 845 SE and can't say that based on my experience, I could recommend the 518 to anyone.

I have a 501ia with aftermarket (but certified service) sub output.  I’ve had for roughly 2 years and love it.  I will likely upgrade the tubes, but it really has a warm sound to it.  I have the Harbeth HL5 plus speaker and while i love them, you are right they recreate high frequency very well with the super tweeter and that is why I’ve had to add the subwoofer to help bring better balance.  
I want to replace some tubes and upgrade my turntable from a Rega P3 trying to tease out a better mid-range.