The proof is in the listening, but generally speaking series-connected NiMH batteries are a poor choice for SOTA. They have up to several ohms of impedance, as compared to several milliohms for large SLA batteries. You can hear these differences, though adding massive capacitance may ameliorate the situation somewhat.
Why would a designer choose NiMH over SLA? Convenience, ROHS issues concerning lead content, gassing issues, transportation issues. But not for SOTA.
Why would a designer choose NiMH over SLA? Convenience, ROHS issues concerning lead content, gassing issues, transportation issues. But not for SOTA.