What sub for music budget

I have a pair of Monitor Audio 3i's on the way, to be running off a NAD T750 and C523. For my electronic music, I think I will want a sub just so I can hear the bassline (usually sub-50), though it'd be nice to have for all music.

Any suggestions on which sub I should get? My budget is not huge, as is probably obvious considering my choice of other gear, but I'd like something musical.

I recently listened to some subwoofers at GG's, and only hated what I heard. They sounded inarticulate and overly deep (like they didn't play as high as necessary, and the frequency response actually went up the lower the bass got. There must be subs out there made for musical listening, I'm just not sure what their names are.
And for the record, though I listen to electronic music mostly, I like an even sound. I don't like my highs too bright, and I don't want my sub to be overbearing.
In addition to ambient, drum & bass, dance and trip-hop, I also listen to jazz, classic rock and the occasional hip hop disc.
Dayton kit or DIY with hypex plate amp or Adire kit or DIY with same amp.
A significant part of my music collection is either electronic experimental or electro-acoustic music. Trust me, you DO NOT want a cheap sub because it won't be able to adequately reproduce a huge part of material you're going to listen to. As a matter of fact, in your case I would wait and invest in a the best sub you can afford, it is not where you want to cut corners. Initially, I tried a HSU VTF MK2. While powerful and able to shake the foundations of the house, it lacked the ability to reproduce bass musically. A few subs of note: Alon Thunderbolt(I think it's still available here on A'gon) for around $700.00.) ACI(Force or better yet, Titan).
REL. Vandersteen 2Wq. I ended up getting a Soliloquy S-10, partly because it matches my speakers, and it's fantastic!.
From my experience, for electronic music reproduction, you'd want to avoid placing it anywhere close to a wall(it would interact too much with room acoustics and distort the bass). Front-ported subs would work the best. My ideal position is on line right between the speakers, aproximately 5 feet from the wall. Hope that helps.

A used HSU VTF-2 is a bargain at $300-400
A used REL strata runs about $700, but if you ever sell it you'll get almost all of your money back.

The problem with most budget gear is it becomes worthless if you ever resell it.

the REL will survive many system upgrades. I run mine placed according to Cardas (out in the room, uneven distance from side, rear walls) and crossed with the main speakers at 45Hz. I don't listen to electronic music, but I do listen to a lot of large scale classical and organ works and the sub blends perfectly. Kick drum has kick, not boom and deep organ notes will make everything in the room shake.
For a good musical sun at <$500, look at the used PSB's. Very nice at the price point.
Try a Totem dreamcatcher sub.
Check out the ACI Force. Aci is know for subs that are musical an big big bangs for the buck. They also have a 30 day money back guarantee