Bass rumble performing in enclosed band stand

Hey gang:

    Our 5 piece Polka/classic country group started using the L1 (2) systems last summer with great results. We eliminated the heaving Peavy PA system and 3 heavier amps as well as improving the clairity of the lyrics and cleaning up of a variety of instruments from accordian to bass guitar to banjo and flat top. Last Sunday we performed in an older dance hall which had a stage recessed on three sides and it raised havoc with any bass tones either on piano or bass guitar. We finially got a bass amp out of the trailer and did the gig as voices sounded great but any low tones had a flutter eco. Normally we set the "stove pipes" behind us but this time to improved sound distribution we had then up front. Any ideas as to why this happened. I would be interested to hear if anyone else has experienced the same problem. Otherwise we love the system and have heard repeatedly from our dancers that they can now understand the lyrics. 
                           
                              Thanks, Larry   (Polka Muzicmaker)