When i tried it first the metronom sound was also played back by pianoteq , so i posted a question about that problem at the Sibelius Help Center and got a step by step explanation from the support two days later. Have a look here:

http://www.sibelius.com/cgi-bin/helpcen...;groupid=3

Yeah, that #$@#@#!@%#$%@ metronome, turned "on" by default.  That thing's bloody obnoxious!!!  (And it was a pain in the neck to finally disable.)

which version of Sibelius do you use? VST and AU Plugins are supported since Sibelius 5.
I guess you want to recall Pianoteq as your default sound every time you create a new piano staff in the score? That is possible but there are some steps to get there. When i tried it first the metronom sound was also played back by pianoteq , so i posted a question about that problem at the Sibelius Help Center and got a step by step explanation from the support two days later. Have a look here:

http://www.sibelius.com/cgi-bin/helpcen...;groupid=3

Best regards,
Robert

I haven't actually used Sibelius in a little while now (I'm going for MIDI-only and/or graphic notation these days), but that was my main reason for initially getting Pianoteq!  (So I could go all "Sorabji" with it!  ;^)

Just open the mixer and select Pianoteq as the playback device.  (IMHumO, this is one area where Sibelius could be a bit clearer.  Too much crap is buried beneath the surface...)  You may have to configure the "playback devices" first.  (I've uninstalled the program, so I can't do a quick "open & compare" for you.  But the manual explains enough for you to get by.)

I do not believe Harmony Assistant hosts plug-ins (and, quite frankly, it's a bit "behind the times" now), so that would be a dead-end.

In other words, how do you connect them with your other instruments? Or are you thinking of getting all of them at once, and are worried about everything working together?

I work on a Macintosh.

Regards Leander