As I said, the speakers are ok but there are probably better speakers (for an HTiB) that come with other mfrs, but only slightly better. Our room is about 12x17 with a 12' cathedral ceiling and the sound fills quite nicely. Wood flooring so there is little sound absorption. The YHT-397 and 399 use the RX-V373 receiver I think. It's the same as the RX-V371 only it has an auto-calibration audio feature. I'm not sold on any the auto-calibrations options that come with the speakers on any of the HTiB's. I'd rather manually calibrate that let the receiver calculate the room response for the speakers. Bigger, higher quality speakers probably benefit more from the auto-cals (Audyssey for example).
As far as speaker models that come with the above mentioned YHT's, you'd have to look at the specs and see if they give the speaker model numbers. I know that mine came with the NS-AP2600's for the front sound stage and the SW012 for the sub-woofer. Keep in mind that speaker specs can be misleading somewhat because you don't know how the given specs were determined (anechoic chamber, etc), and what is on paper will differ from what you hear depending on your listening environment (furnishings, floor coverings, etc). Again the nice thing about the Yamaha system (and any other for that matter if you shop carefully) is that you can always upgrade to better speakers down the road as your budget and needs change. The Yamaha can handle 6-ohm and 8-ohm speakers quite easily.
Good speakers can make a mediocre receiver sound great, but bad speakers can make a great receiver sound poor.