this came up in an unrelated thread recently.
there are many ways to do it. the "easiest" way, is to nest genres, which you do with the backslash \
so for the rolling stones, you might have Rock\Classic Rock. you can delimit them also like Rock;Blues\Classic Rock;Folk
this will show up in a view as >Rock
>Classic Rock
the downside to this view (for me) is that the nested "sub genres" stay nested...that is you can't work with them and while great for viewing it's hard to work with IMO
you can also create an infinite number of custom fields like [sub genre 1], [sub genre 2], etc. this is a lot of work, but makes the genres "un nested" so you can work with them separately
what i did was use keywords (you could make another custom string field). The nice thing about this, is keywords are really easy to work with, so i'd have for house music
[genre]=House [keywords]=acid;deep;electro;progressive;tribal or, [genre]=electronic [keywords]=house,acid,electro, etc.
this way, i can put genre in one pane, and have 3 or 4 keywords panes, and select as i want. if you keep keywords a little more generic (like "acid" instead of "acid techno" and "acid house", you can use the "acid" keyword across all appropriate sub genres, like
[genre]=electronic keywords=house,acid,progressive
[genre]=electronic keywords=techno,acid,progressive
if i highlight [keyword] = progressive, without any [genre] or any other keywords, i would get rock,metal,and electronic albums...so the keywords can be used across genres rather than nested in specific genres. if i highlight [keywords] = progressive, house, i'll get just that. or highlight [genre]=electronic, [keywords]=progressive, i might get progressive house, techno, dubstep, etc.
this has taken (and continues to take) a lot of work, but i'm really enjoying the power it gives me, and it's easy to pane tag keywords with the checkboxes so it's good for workflow. i have about 20 actual genres, and around 250 keywords so far, but by keeping my keywords somewhat generic, it makes them powerful over all genres