Audax Home Theatre Again

I'm planning to build the home theatre system illustrated for the Audax website. The designs for the centre and rear speakers are perfect for my needs, but i would prefer the front speakers to be narrow floorstanders. If i maintain the net internal volume of 30 litres, but change the dimensions, will the sound/response be dramatically degraded?

Any advice on this topic would be most appreciated.
In theory the sound should stay the same, but this is not always the case. When you change the dimensions u are altering the reflections internally of the sound waves. This shouldnt come into play if your cabinet is well dampened but nothing can be guaranteed. Its a bit of a trial and error thing.
The larger question is, how much does the baffle affect the response curves. Ideally, i suppose, most drivers should be mounted on an baffle of infinite size, so the drivers are radiating into half-space with no diffraction.

In the real world, baffles are obviously, of finite size. this leads to problems with acoustic diffraction. on a practical level, there are primarily two kinds of diffraction that you need to worry about.

the first is spreading loss, or baffle step loss, that audiofreak mentioned. this is usually a 6 db drop in the lower midrange that occurrs as the LF driver radiation goes from half space to full space.

the second, edge diffraction occurs when the sound tries to wrap around the edge. a pressure drop occurs which causes a secondary wave of opposite phase to get radiated in the forward direction. this creates ripples in the frequency response curve.

i suspect that dr d'appolito, when he designed the speakers, built the baffles, made the measurements, then designed the xover to minimize these effects.

if you change the baffle size, you will affect this. unfortunately, usually making the speaker more narrow, i.e. more socially acceptable, worsens diffraction.

how bad will this be? hard really to say.

I don't think that internal standing waves will cause too much havoc if you follow the standard rules of box dimensions, construction, etc.
Provided you can clear up these low frequency (remembering that you really don't need to extend below 80Hz for any channel but LFE in home theatre) problems, that narrower speakers will help improve tweeter and mid dispersion. Also consider replacing Audax's tweeters with something like LPG 26Ts (I'll have to say one of the best tweeters I have ever heard.)