The ideal way to do this is to line out of the power amp for the guitar and to plug the Tyros directly into a channel. In this case you're going to need a powered mixer, or separate mixer and power amp. There are a lot of small powered mixers. I use a 6 channel 200 watt Carvin with onboard effects, but hear good things about Beringer. You can't just plug your Tyros into a non-powered mixer and then into your speakers, unless they are powered speakers.
Expect to pay between $200.00 and $400.00 up for the equipment you need. Gary can also help you if you're interested in the Z-5500 or similar unit. You'll need a simple non-powered mixer (at least two channels for instruments-more for vocals) in that case. While some of us (me included) don't like this approach, that may be the least expensive way to go.

Hope this helps.


Russ