A one piece sloping ceiling (ie not stepped) joining the two spaces would be an even better option from the acoustic perspective.

By increasing one of the room dimensions you will spread out the room resonance nodes across a greater frequency range, with each now much weaker than before.

If you google you'll discover ideal room dimension ratios which have been calculated with this in mind.

With the extension to 15ft you'd have roughly 3:4:5 which rings a bell from somewhere deep in the grey matter as being a good thing.

You must also think about the floor though - floor boards over joists create spaces in which sound will resonate. Look to wall mount your equipment for isolation, fill those under floor voids with loft insulation to increase the air density and consider very dense plinths (granite / marble) underneath the speaker.

Unfortunately this project is going to last longer than a weekend! Are you sure that headphones aren't the way to go?

(If you're up for a brain scramble http://www.linkwitzlab.com/modes1.xls)