An answer on this question Suggested method for boosting 0.05 watt transmitter up to 10+ watts? suggests putting a filterless 19dB amplifier between the HackRF (or similar general-purpose SDR) and a larger amplifier — so only the final stage includes much in the way of filtering.
Is some caution warranted here for spurious emissions?
As far as I can tell, the HackRF's own filters are before (from a transmit perspective) its RF amplifier stage.
The project's block diagrams aren't very helpful in this regard, but http://www.wimo.com/hackrf-sdr_f.html has a detailed write up ("After the antenna input two MGA-81 GaAs MMIC amplifiers follow, one serves the input, the other the output path.") and provides this frontend block diagram:
So after its filtering, the HackRF adds a +14dB gain of its own. After which, we might add something like the PennyWhistle, QRP-PA 2008 or PA-100 to boost the SDR's still-small signal closer to the 5W range — something like another 15–25dB further gain. Finally we might put this through something like the HF-Packer or HARDROCK-50 amplifier which provides another ~10dB gain, before the signal finally getting filtered again by that amp's LPF section.
Is all this gain going to add more spurious emissions than the power amplifier's LPF section is likely designed to handle? I've been told that one really should filter after every stage of amplification, but in this setup the signal goes through three stages before filtering. Perhaps the "pre-amp" stages are much easier to keep clean/linear, or is that a bit too optimistic?
How could I use the specifications for each of the pre-amp stages to determine all the expected types and levels of spurious emissions they are adding? Do these harmonics compound from one stage to another, i.e. if the input to the external pre-amp is already not clean how do I take that into account when calculating the distortion it will add?