When you say "Studio Monitors" I'm guessing you mean "Powered Studio Monitors" that have their own power amps built right inside of them, right?
If so, a tape record out can work too. If you only have the power amp output that usually goes to a speaker, you can use an "L" attenuator. This will match the signal level and impedance to the line level input of the powered speaker. I found this site: www.epanorama.net/circuits/speaker_to_line.html
Powered speakers are of course very popular but you can still buy Passive Studio Monitors if you would prefer an easier hook-up.
The Avantone speakers are available in passive models including the Mixcube and the CLA-10. The mixcube is only reliable from 90-10kHz in my room but the transient response is much better than that of my Yamaha HS50. Never used the CLA-10 nor its forebear.
Speakers are not a subject of love or worship for me. If they don't stand above their surroundings, then it's better. They are nice, for example: Linn Tukan, Phonar Veritas M4 Next. With new ones Yamaha monitors look cool. E.g HS Series, black.
I remember Ethan mentioned his choice was Mackie 8" monitors so I wanted to enter them into this discussion even though I don't own these (I have MR5's ... it's a space issue, $$ too (sigh)). Here's a thread I found. Here you'll find, Hexspa explains some details of things to consider. the-audio-expert.freeforums.net/thread/573/6-8-monitor.