Have you considered a Transmission Line design, say driven by the Kef B139.
T-line speakers extend the bottom end often to below 25Hz. Here's a useful
link about T-lines:
http://www.t-linespeakers.org/index.html

I use IMF TLS80's which is a 30 year old speaker, but still sounds good to
me. You might get some of these on ebay. Here's a link:
http://www.marklev.com/IMF/

You could also try Linn Isobariks which use two bass units in push-pull with
again very low bass.

Good Luck,
Pete

I've currently got an Arcam Alpha 3 amp (40WPC) running a pair of Cresta
2s.

Has anyone any ideas on speakers to replace them with, preferably ones I'm
liable to pick up second hand. Floorstander VS Bookshelf doesn't really
matter. What I'm after is better bass - the Alphas don't seem to produce
good low end bass, which is hardly surprising considering the size of the
box and the drivers. Preferably something that isn't a bass reflex design
as
they're not straight in front of a wall. I might be better off with a sub,
but I rarely hear of anyone using them for music rather than films except
for Don Pearce's post just now.