The Q700 will be double the price of Q300. I feel, if the cross-over from the speakers to sub is set around 50 Hz, then I would not gain by going to floor standing ones. The Q300 is rated from 42hz to 40khz and the Q700 is 36 hz to 40kHz. My main use is for movies.
This is the problem with spec sheets.
the Q300 has a single 6.5" driver covering the range from about 42khz to 2.5khz. It's a coaxial speaker, so the midrange cone reflects the rearward response of the tweeter forward, and when it's moving, it's modulating this reflected response.
First of all, a crossover at 50hz is too low as it is. Run a 50, 60hz tone at a decent SPL through a coaxial, and unless the vent is dominating the response, you get a lot of motion. This will affect the response in the 2khz to 6khz range!
So you will want to cross over either of these speakers above 80hz,
Even at 100hz, you get a lot of driver motion from a 6.5" cone.
The Q700 has twice the surface area, which will HALF the driver motion at the same SPLs. That will mean more max output (more realism on dynamic sequences), better sound at the same mid-high outputs(less distortion and modulation), and thus a smoother transition from subwoofer to midwoofer (because one hasn't run out of steam, giving itself away).
I would cross either of these speakers at 80hz to 100hz because they're 2.5 way coaxials.
Just because the spec sheet tells you it goes deep, doesn't tell you how it behaves when it goes deep! And even when crossed over at 80hz,
behaviour from 80hz to 300hz is very important.
