I can only offer my experiences with the ARC gear as I have not heard the Pass pre amps.
The LS16 actually has a higher gain output of 12.2db in SE config. The 25 is adjustable from 0,6,12db Se.( 6,12,18db in Balanced) Your 15 runs 6db in SE out,so the 25 would be a good choice, run at the lower 0db. setting.. That said , I dont think that the 25 offers any more flexability with volume as the stepped attenuator is much as the 15 is, however the lower gain setting will be useful for you here.
Sonically, the 25Mk1 is a nice improvement over the 15 in every area, and cost about 2.2K used. compared to about 2.9K for the used Mk2 which incorporates the 6H30 tubes over the 6922's. For me, I prefered the Mk1 over the Mk2, as it had more airy mids and highs but slightly lighter bass than the Mk2.Not a big difference here, but worth noting. I thought the 6H30 tubes were almost too "accurate" sounding, they are very linear and provide good dynamics, but the Mk1 was able to bloom more in the mids..
The LS16 actually has a higher gain output of 12.2db in SE config. The 25 is adjustable from 0,6,12db Se.( 6,12,18db in Balanced) Your 15 runs 6db in SE out,so the 25 would be a good choice, run at the lower 0db. setting.. That said , I dont think that the 25 offers any more flexability with volume as the stepped attenuator is much as the 15 is, however the lower gain setting will be useful for you here.
Sonically, the 25Mk1 is a nice improvement over the 15 in every area, and cost about 2.2K used. compared to about 2.9K for the used Mk2 which incorporates the 6H30 tubes over the 6922's. For me, I prefered the Mk1 over the Mk2, as it had more airy mids and highs but slightly lighter bass than the Mk2.Not a big difference here, but worth noting. I thought the 6H30 tubes were almost too "accurate" sounding, they are very linear and provide good dynamics, but the Mk1 was able to bloom more in the mids..