The instruments that comes with MainStage is no problem at all, but it can't handle Pianoteq... I haven't had time to test other 3:d part instrument yet (I don't have any installed at the moment). So I don't know if the problem is in al 3:d part au-pluggin or only with Pianoteq???

Anyone else that have checked this out???

The problem with older applications is, they are not supported anymore and probably they don't run on the actual OSX.

In the meantime I tested DSP-Quattro Demo, and I think for me that could be a solution.
It's very simple to arrange my Instruments and Effects and the response is better by far than in Mainstage, no dropouts, no crashes.

Going for the full DAW is perhaps an overkill if you just want to perform, but if you have to, Reaper is very attractively priced.

The best way to do what you want to do is to get yourself a so-called DAW, a 'digital audio workstation', which is a virtual multi-track recording and editing environment for all things audio and midi. On Mac, the most obvious choice would be Apple's Logic (which you can buy in, and download directly from the AppStore), but there are quite a few other choices as well (Cubase, DigitalPerformer, StudioOne, ....). They're all pretty good, each has its own passionate crowd of users, so you might wanna ask around a bit (and maybe look at a few presentation videos on YouTube) before choosing one.
But if you're serious about making music with multiple layers of instruments (combining Pianoteq with various other instruments), you really do need a DAW.

Having said that, you should also know that changing to a DAW will probably not solve those CPU-problems. If your hardware can't quite cope with rendering Pianoteq successfully in Mainstage, it's very likely that you will face similar, possibly even worse problems when moving over to a DAW.  The only solution here is to upgrade to a more powerful machine, I'm afraid.

If you're not really interested in multi-track recording, but are mainly looking for software which allows you to play one or more instruments live, maybe DSP Quattro might appeal to you. DSP Quattro started out as an audio editor and that's still its main business, but it also does 'plug-in hosting', i.o.w. it allows you to load one or more virtual instruments (and process their audio output with various effects, should you like). Seriously good software. But not a multi-track recording/editing environment, like those DAW's are.

Anyway, if it were me, starting out on Mac today, I'd first get the most powerful Mac my money can buy (with as much RAM as possible) and then add Logic it.

_

I sometimes use different standalone applications when  I'm working on kontakt patch and I want to test it layered with PTQ or other plugs; I never had a problem but I didn't check it in a live situation honestly.

I'm new in the virtual Instruments world and I'm impressed by the functionality and possibility of Pianoteq.

I like to use Pianoteq,  NI's Vintage Keys and some other Kontakt Instruments live.
I've tried out Apple's Mainstage, but it has CPU overloads and crashes while playing arpeggios with sustain pedal down, it's really disappointing.

Is there an alternative to Mainstage??
How do you play layers, for example piano (pianoteq) and strings or piano and a synth pad together?
Is it possible to run both stand alone applications (Pianoteq AND NI Kontakt) at same time on several Midi Channels?

Thanks for any suggestions
Urs