Yeah, I agree absolutely about dealers having speakers for you to actually listen. That's the whole point of having the dealers. Otherwise, as you say, might as well buy ID.
Which ID speakers have you owned?
The SX-6300 & SX-8300 should sound identical in the midrange and treble, all things being equal. Same with the F206 vs F208.
The advantage of the SX-8300 or SX-6300 is that you could actively bi-amp the bass section using a subwoofer amp (like the $300 HSU subwoofer amp) and listen in stereo 2.1 mode (LFE output).
If you think the bass of the SX-8300 or SX-6300 is too much for the room, you could easily reduce the bass like you would reduce the bass of a subwoofer by just turning down the volume knob of the subwoofer amp.
One SX-8300 tower has the bass equivalent of one SX-1010 subwoofer. For most rooms that are 18' x 20', you would not need any other subwoofer since dual SX-8300 is like having dual SX-1010 subwoofers.
Bottom line, definitely audition at least 2 different times before you buy no matter which speakers you end up buying.