My mate Tony came over to hear the Lancasters and we tinkered a bit before he suggested that I move the speakers as far apart as possible in my room. To my surprise, that made quite a difference - the Lancasters still have a bit of that 'plodding' character, but it's reduced and there is a much better soundstage and more believable top end. So, I have spent the last few hours carefully dismantling the system, dusting and hoovering everything, and rearranging the rack, furniture etc. To be fair, it was worth it for the Spring Clean!
So, the next step is to make up some replacement crossovers and to rewire the speakers internally. I am trying to find out the optimum value of the main HF section inductor, but all the other bits are reasonably inexpensive from Falcon Acoustics.

