I also have the SD with the green-ring mic. head - but you are really pushing it too far with what you are recording.
For what you are doing you really need quiet mics with a high output and a top-end mic. pre.
The Nagra SD with the green ring is a great little portable unit for all sorts of things - but for what you are doing I replace the mic. head with the XLR line input cable and use a Nagre EMP or AETA MIXY with good mics (eg: Sennheiser MKH 8000 series).
Still light and portable, but you are pushing it too far with using it as you are.
Thank you John for your feedback and advice. I guess I needed someone to tell me that my expectations were too high.
I do like the SD for what it is and I was thinking about using it as a line-level recorder. The gear you mention is outside my current budget I'm afraid, and since the recording part of my working with sound is not the main aspect of my practice (I am a composer), I'll rarely have the means of buying even one new item.
I've gone through my share of gear over the years. Currently the best mics I own are a stereo pair of Oktava MK012 with the premium modifications from Bill Sitler and exchangeable capsules. I have made some (in my humble opinion) pretty decent field recordings with these using a self-made shockmount, basket and windjammer, going into a Marantz PMD661 (the latter of which is out of action, hence the purchase of the Nagra).
I am now hoping to upgrade my gear bit by bit over the next while, starting with a Rycote windshield kit, using the Oktavas as a MS pair with their ∞ adapter. The question is, in terms of preamps, what would be my best option, for ca. €1000 or less, to power the mics and bring them into the line input of the Nagra? Would I go for something like the SD MixPre? Any further advice would be much appreciated.