Apologies for the length of this post. I have quite recently had
serviced and put into operation a fairly old valve amp and I'm excited
and confused by it. I hope you can help.

It’s a Beam Echo Avantic Type SPA II. It sounds great to me. I
should say I probably don't have the same standards as most on this
newsgroup when it comes to audio - prior to this I was happy enough
with my old NAD 3020i which, while I knew it could be better, was fine
with me. I don’t know if the Avantic is higher quality but it
definitely sounds different to the Nad and I much prefer it. I would
love to know more about this amp and I can find practically nothing
about it on the web or anywhere else I’ve looked, so I'd appreciate
any information and opinions anyone might have. I know Beam Echo was
a British company which I think wound-up sometime in the 1960's. I
also remember reading some time back that this was not a particularly
high-end bit of kit in its day and not particularly well thought of,
Beam-Echo produced other, more expensive, amps. The person who
serviced it for me says it was built in 1959 but couldn’t (or didn't)
tell me too much more. Any knowledge you can share would be very
welcome.

Also if anyone can answer any of these questions I would really
appreciate it:

1) On the back is a dial labelled PU Balance which turns 360
degrees. PU is the phono input but what does this dial do? When I’ve
adjusted it I can’t hear much or any difference but I am nervous about
breaking anything and haven't fiddled much with it.
2) Another dial on the back has an arrow you could set to points
labelled 4, 8 or 16 ohms. But the dial doesn't seem to work, does it
matter?
3) It has a dial on the front labelled Balance but this seems to do
very little, certainly doesn’t act like the balance dial on any other
stereo amps I’ve used which move the sound from one speaker to
another. What is it meant for?
5) The source selector dial on the front has 6 settings - on one side
PU, Tuner and Tape under the heading stereo and and on the other side
the same 3 input choices headed 3D. All six work but can anyone tell
me what the 3D setting is for? I can't tell much difference between
them. If pushed I'd say the stereo settings sound better but that
could just be my imagination.
4) It needs a phono amplifier – I use it with a Thorens 160 MkI with
Shure M75 – do you have any recommendations for a decent one in the
not too pricey range. Currently I have the turntable plugged into the
Nad and the Nad pre-amp outputs plugged into the PU inputs of the
Avantic and I wonder if a dedicated phono stage will make a worthwhile
difference.
5) Finally, any suggestions for an improvement on the Linn Index
speakers I'm using. I understand the Avantic is fairly low powered
and that I need speakers with a reasonable sensitivity (the Linns are
rated 87 or 86db). I don’t understand what the sensitivity measure
means and for what its worth it goes very loud - but always (for me)
with something missing in the bass sounds. Is it worth going for
expensive speakers to improve the bass and generally sound better or,
with this set-up, will it not make an appreciable (by me) difference?

Sorry for being so long-winded, and dim.

Thanks in advance for any help...

Dave