Condition is unknown, and at US$650 this sits well above the recent-used median of US$180 and even over the 75th percentile of US$350, so it reads ambitious unless the unit is exceptionally clean, fully tested, and includes the right install details. For a buyer, that means this is not a casual impulse buy; the price only makes sense if the seller can justify it with strong provenance or unusually complete presentation.
What makes it worth considering is the Sonamp line’s reputation as a serious custom-install multichannel amp: lots of clean power, stable day-to-day operation, and a format that still appeals to whole-house and theater setups. If this one is truly healthy, it could be a practical workhorse, but at this ask the buyer should verify channel functionality, any rack-mount hardware, and whether the fan/noise profile suits the intended room.
Independent perspective — not a price guarantee. Always verify condition, accessories and provenance before purchase.