I've never heard this sub, but from looking at the specs, don't expect much below 42hz. This should be 'barely adequate" for most music, but not so much for movies.
What are your goals, what other subs have you heard, and how much is he charging you? And what are the other speakers you'll be using?
And what makes you label him as "good" ?
According to the Jamo's owner's manual (PDF):
- 6.5" driver
- dual 6.5" passive radiators
- 800 "mystery" watts amp
- 42-200Hz +/- ?dB (the product page says +/-3dB)
- 8" x 8" x 8"
Dunno what your installer is charging you for it, but it's listed on Amazon.com for ~$320. For $300 or less when it's on sale at Newegg.com (at this time, it's selling for $279), you can snag a Klipsch RW-12d which will easily outperform the Jamo.
Have the installer take back the Jamo, if you can. If you can't, well, I hope it rocks as well as he says it does.
I have two of these in a theater in opposite corners from the listening area and they are loud. I loved their small footprint and piano white finish. They replaced my velodyne 12 and I have never regretted it. They are very clean in sound.
I also have two of these (or rather the Mirage version) in a home theater/music setup that I chose to keep compact. I'd say they are ok. They are a little loosey goosey so with music I find the bass to be undefined and lack transient response. I think the loose nature helps in a home theater environment because it adds to the rumble.
I'd recommend them maybe if you really need small size but if you have room, I am confident you can do better for the money.