A computer measurement system is the best way to evaluate your room modes.
http://www.hometheatershack.com/roomeq/Once you have a scan of the frequency response, you can post it here for specific advice about how to flatten it.
You can use ModeCalc to predict and to help analyze the cause of modes that you see on the scan. This can be helpful in finding efficient solutions.
http://www.realtraps.com/modecalc.htmIf the Westminsters have too much bass, there are ways to fix it, but you would be better to pick a smaller Tannoy that matches your room size, like Definition T8. A modern living room is usually one of the largest rooms in the house, so it may not be as much of a problem as you think. If it opens up to other rooms and hallways, and the room is standard wood and wallboard construction then you should be fine with the Westminster. If it is a sealed room (closed doors) less than 2000 cu ft then it might be too much.
If there is too much bass a 1st order line level passive high pass filter would roll off the lows with a resistor and capacitor. You would put it between your preamp and your amplifier, or solder it right into the output jack of your preamp, or your interconnects. I think marchand makes such filters, but it is so easy, you can diy a filter for yourself, then you will know how to tweak it to perfection.
http://www.t-linespeakers.org/tech/filters/passiveHLxo.htmlThanks to Dave Dlugos (planet10) for hosting that page.
There are active solutions to attenuating excess bass that would also allow for tuning the bass to match the room modes of the room, like miniDSP.
