Blindjim - the main frequency "amplified" by your room will be 1125/(23x2)= 25Hz. Unfortunately lack of absorption creates standing waves with peak and valleys at different frequencies at the different location. To tame it you'll need at least 4" high density material placed some distance from rear wall. 4" panels are not common and are expensive. Better to concentrate on removing higher bass resonances. To do that you need 2" thick 4'x8' panels of high density fiberglass like John Manville 817 http://www.jm.com/insulation/performance_materials/products/ci9_800series_spin-glas.pdf
814 is more common but has much worse absorption coefficient at the low frequencies - I ended up buying twelve 817s.
I was once in the room that had all spaces between studs filled with 814 or similar Corning material with fabric over it. Amazing acoustics - you can hear voice so clear and pinpoint exact location/person. My panels are in the making - got backboards, fabric glue etc. but am too busy (read lazy) to finish.
Owens Corning might be more common in your area:
http://esh-docdb.fnal.gov/cgi-bin/RetrieveFile?docid=1195&version=1&filename=Owens%20Corning%20700%20series%20fiberglass%20insulation.pdf
814 is more common but has much worse absorption coefficient at the low frequencies - I ended up buying twelve 817s.
I was once in the room that had all spaces between studs filled with 814 or similar Corning material with fabric over it. Amazing acoustics - you can hear voice so clear and pinpoint exact location/person. My panels are in the making - got backboards, fabric glue etc. but am too busy (read lazy) to finish.
Owens Corning might be more common in your area:
http://esh-docdb.fnal.gov/cgi-bin/RetrieveFile?docid=1195&version=1&filename=Owens%20Corning%20700%20series%20fiberglass%20insulation.pdf