The 410UL is a classic, and if you like that wide tone with a bit of scoop in the low mids, and that 'space' between the top of the driver and tweeter, it is great. Gigged one for over 10 years as my primary cab.
The 502 was another thing. Very strange amp... plenty loud, TONS of low end, but an unusual mid voicing and very limited tone controls. I really, really had trouble with that head in many rooms.... it went from to bassy to too thin with just a touch of the knob. Also, even with the treble control shifted down, I found the treble response very 'hi fi'... clean and clicky... sounded great in the basement, disappeared the first time a cymbal hit. This is why many describe the 502 as a 'dark' sounding head.
Both the Markbass heads, especially the F500, and the Genz heads, especially the Max 6, sound wonderful with the 410UL... punchy, tight, articulate, but still plenty of bottom, and LOTS of control over the mids.
If you want that more super clean, super wide, super hi fi type tone that the 502 attempts to achieve, the TecAmp Puma500 is a much better options IMO... similar voicing, but MUCH better EQ control over the mids, and a more usable treble response.
IMO and lots of IME.
Edit: Have not heard the 501, but it looks like basically a single channel SMPS version of the 502 (which had a toriodal transformer and class D amp).
