I have been playing around with trial 4.5 version on Linux. I'm running Fedora 19 32-bit version -- binaries ran without me needing to install any software but I already had a bunch of ALSA/MIDI libraries installed for PianoBooster.
I'm using a spare laptop -- a pretty old one from 7+ years ago. The CPU in it is a Pentium T2310 @ 1.45ghz -- roughly a Core 2 Duo without hardware virtualization -- so really it doesn't have enough power to run Pianoteq at maximum settings. In fact, the first time I started testing it, sound cutoffs and distorted sounds was 25% of the notes.
A few things I've been able to tune to get it working better under Linux:
* Do not use PulseAudio or JACK. If you don't need JACK (to connect to other MIDI software), it's better to output sound directly to the hardware devices. (On this laptop, it shows up as "HDA Intel, 7.1 Surround Sound" -- look for something similar to choose.) Otherwise, the audio servers themselves take up a lot of CPU power.
* Ignore the recommendations for RT kernels. For MIDI latency, regular kernels work fine and don't take the RT kernel performance hit.
* Minimize the window when playing -- that reduces cpu usage by roughly 5%.
* Use crontab to renice the priority if your user account does not have sudo rights to renice. Here's what I have:
* * * * * ps -ef | grep -v grep | grep Pianoteq | awk '{print $2}' | xargs renice -10 -p > /dev/null 2>&1
* Set hardware sampling rate to 48KHz, internal sampling rate to 24KHz.
* Use the "intimate/player" models with less reverb.
* Set polyphone to Auto (pessmistic)
* Set buffer size to smallest amount for lower latency.
With the above options, Pianoteq is very enjoyable to play. It makes me want to hit the ivories/ebonies all the time for practice.