Subs And The Downstairs Neighbors

I live in a brownstone in Fort Greene, Brooklyn. The building has the usual suspended hardwood floors but there must be magic insulation because I can listen at 90-plus decibels and remain quite friendly with the couple downstairs. Nor do I hear anything from upstairs.

I have an Arcam A19 and a pair of LS50s. The system sounds great but one wouldn’t call it chest-thumping. So I’m thinking about a subwoofer.

In traditional New York style, my apartment is both small and expensive. So I’m looking for a cheap, low profile, downfiring sub that will fit under the couch. Monoprice? I’d rather get something used but the only other low profile model I can find is a $1200 Focal.

The secondary question is whether I’ll be defiling the sound of the LS50s by using a cheap sub. I haven’t heard the Monoprice but it’s probably doesn’t sound REL-like. Supposedly it reaches 30 db. Is that low enough? The sound doesn’t have to be great, it just has to be better than it is now.

The primary question is whether the sound will break into the downstairs apartment. Just a gut reaction but it seems like I’d be asking for trouble.

There’s an similar thread from 2008. The poster only got two responses from people with actual experience. One nearly got evicted, the other had two subs and reports that all went well. I thought I would check to see if anyone in a similar position has joined since then. Anyone?