The cheap yet "elegant" way to deal with this is, indeed, based on
a "dimmer" switch type of circuit.  These usually contain a triac which
is a solid state electric switch.  The circuit contains a resistor and
capacitor network which causes the triac to switch the mains voltage
off part way through the A.C. cycle.  It automatically switches back on
when the AC alternates.  The variable resistor (dimmer control) alters
the point at which the triac switches off, thus changing the effective
average voltage on the motor.  (The switching happens 120 times a second
in the US, 100 in the UK).

Anyone with a basic understanding in electronics should be able to
add a second resistor-capacitor network, and another variable resistor,
which can be switched into the circuit by an external microswitch.
Set one control for the playing suction level, and the other for the
re-wind suction level, mechanically link the microswitch to the rewind
lever, and you have a semi-automatic system which reduces suction to
a pre-set level on rewind.  This is the system used on "commercial"
suction boxes in the US, which also use universal motors identical to
vacuum cleaner motors.

Most modern vacuum cleaners (at least in the UK) are rated at 1000
watts or more, although you would probably only ever draw half that
if the speed is lowered for a player in reasonable condition.  I would
suggest a minimum rating of 500 watts, preferably higher, for the dimmer
unit.

The concern about the switch burning out is because lighting loads are
usually resistive, whereas motors are "inductive" and can cause high
voltage spikes when switched off, overloading a lighting switch.
However, since we are talking about running the motor at much lower
than its rated speed, the risk is reduced.

Triacs can cause over-heating of motors because of the high frequency
switching, but most vacuum motors will have some sort of internal
protection which will disconnect them if they overheat.  Make sure it
doesn't run for long periods with no airflow (i.e. sucking but not
playing).

John Farmer, UK
www.musicanic.com