I have not heard the SPL, I was considering it at one time. I already had a LA4 and needed another preamp for another system I was building.
In terms of being quiet I think the LA4 is going to be hard to beat. You should compare the measurements to verify. The LA4 runs warm which is usual for a Benchmark component. In terms of sound, it really does not have any. If your other gear sounds good, then the LA4 will let you hear that other gear without decorating the sound. The volume clicks and I like that. It is also the best volume control I have owned. All amps I have tried with is work perfectly, I cannot say the same for another LA4 like preamp, the Topping pre90.
If I was not going to get another LA4 (for variety), I wanted another preamp that had similar attributes of the LA4, and I got it with the Holo Serene preamp. It is a touch warmer than the LA4 and the volume is not as robust as the LA4’s. However, it does sound very similar to the LA4 (almost no sound). It is likely in-between the SPL and the LA4 in terms of sonics. I think the Serene works very nicely with Class A amps.