Measuring Pre-Amps

["zeroplusinf" zeroplusinf]

Hello naturerecordists,

I am interested in starting to do ambiance recording as a hobby (potentiall=
y with the intent of commercialising it). Admittedly my technical knowledge=
 of recording equipment is still limited.

The recorders I had a look at so far are:

Fostex FR2 LE
Marantz PMD 661
Sound Devices 700 series

The Sound Devices is an outlier in terms of its price - is it worth spendin=
g this money?
I understand that the major difference between models lies in their pre-amp=
s. I ask myself how the quality of the pre-amps is measured?

Thanks a lot in advance for your help!

All the best

Markus