Acoustic instrument amplification is still a nacent field with a myriad of solutions and no silver bullet. Put another way, there is no near prefect solution such as the wide-diaphram condensor microphone has been in the recording world for acoustic guitars and voice. In fact, many musicians tell me that if you want a true representation of your instrument, go with a tradtional mic/mic stand pointed around the 12th fret and be prepared for feedback.
Even so, electronic pickups have been steadily improving- even in the last decade. For this reason, among others, we do not cut big holes in the instruments and install large 'barndoor' preamps or battery compartments which may be removed later.
Blackbird uses the rechargeable Simple Jack preamp from MiSi, which offers the improved sound and convenience being battery-free mated with the industry standard Fishman Matrix pickup. L.R. Baggs Lyric internal mic systems have garnered quite a following. None of these systems are perfect and Lyric does require a 9v which is easily installed in El Capitan with access through the large sound hole. K&K also makes some nice passive undersound board transducers such as the pure mini, which sound great and don't require preamplification yet still have a strong signal. These however can be more difficult to handle in high SPLs (loud venues like playing with a drummer!) compared with undersaddle piezo pickups like the Fishman.
Finally the image above shows the RMC Acoustic Gold mated with RMC's own Polydrive II preamp for the ultimate in piezo sound with each string resting on a separate pickup. RMC is a bit involved and expensive system, but offers superior string balance and dynamics, with albiet that 'piezo tone' which can be dialed back with the Polydrive. We could install the Polydrive onboard but then what happens down the road when you want the latest greatest version? Especially important for RMC because the Polydrive can also plug into MIDI for an awesome guitar synth work.
All told there are some great options out there now but we'll still see lots of innovation and therefore new form factors and technologies. This means that the current crop of systems will eventually be antiquated with future systems getting closer to the holy grail of a studio level sound with less feedback and easier interface. The instrument on the otherhand will not go out of date (hello pre-war Martins) so keeping the holes to a minimum opens the door to future possibilities.
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