I have been using the APC Back Ups Pro 1400 for several years in my
studio. It puts out a nice pure sine wave and everything can be
connected (except printers of course).

I am a professional instrumentation technician and designed and built
a large telemetry system for a California water agency where we had
over 60 remote sites connected with uhf and spread spectrum microwave
radios. All of this equipment needed to be protected, so we bought one
of every available ups rated at over 650 va to find one suited for our
needs. (some of our sites have over 16 tons of pure chlorine gas which
is enough to kill everyone in a small city, so we really need to know
what is happening even if the power is off)

The final analysis is that the only standby type of ups that didn't
send massive amounts of higher than line frequency harmonics was the
APC 1400 like the one I use. There are others out there like Tripplite
etc, but the quality was not nearly good enough for "mission critical"
applications like ours. I bought a pallet of them and the only one we
had fail was when a 12,000 volt power line fell into our 240 volt
incoming line. The ups was incinerated, but my rtu, radio and a
desktop pc were perfectly ok after we replaced the ups!

I have heard other people say that the non-sine wave inverters are ok
for audio and computer gear, but with falling standards on power
regulation in the USA, there is a new problem where stray medium
frequency harmonics are burning out motor windings and switching power
supplies in large industrial installations, so for studios, I want to
stay away from anything that isn't a sine wave. I have a contractor
friend I work with who is going into power quality as a primary line
of work, and previously, he has done jobs like complete electrical and
automation installs at the new HP Plant, Varian Brothers, NASA
Research, Port of Oakland etc in California. Talking to him really
shed some light on the question of harmonic content on power lines.

Another way to protect the audio monitrs and amps is to build, or have
someone build a relay box with a pushbutton that passes power to an
outlet only after the button has been pressed. If power fails, the
relay opens and will not pass power again until you press the button.

This type of circuit protects equipment from what is termed "rolling
surge". It is when the power returns (usually) that you will see
wildly fluctuating voltages on the power lines because the power
company's circuit interrupters can open and close hundreds or even
thousands of times before stabilizing in the first few seconds of
power being restored.

This also tames the restart bump that happens when power amps are
turned off and back on suddenly.

Tripplite makes a variation of this called the "Power Pause" which is
a plug in timer relay that only lets power flow to the outlet after a
short time out has elapsed, thereby eliminating "rolling surges",
although you might not want the power to come back on automatically...

Hope this helps