The functional purpose of an active buffer is to improve impedance mismatches by reducing higher impedance inputs to lower impedance outputs. This can be helpful when driving amplifiers with low input impedances, when using passive volume controls that do not actively buffer the signal, and/or when driving long interconnects. The effect can be significant, even in cases of unity gain (i.e., no voltage gain between input and output), and the change in sound is sometimes described as having more body, tonal color, and drive. The buffer can be either fully solid state or tubed.
I use the Hattor Audio Big preamp in passive mode to control the volume of my system and it sounds significantly better to me when followed by my SMc Audio buffer (solid state and unity gain) that is based on their VRE-1 preamp. I also have the Hattor Audio Tube Active Stage, which is a tube buffer that typically comes with 9dB gain (mine has 6dB). It is basically a tube buffer stage without a volume control. I have tried the Tortuga Tube Buffer on one of their tours, so I had it in my system for about a week, and I also tried the Purity Audio tubed buffer in my system.
IMO, and in my system, the Hattor Tube Active Stage sounds very good although it just doesn't provide quite the magic of my SMc Audio buffer, so it is currently sitting in a box. However, if I didn't have the SMc buffer, I would be perfectly happy to substitute the Hattor after my passive volume control.
The Hattor Big preamp also comes with an opamp active stage (mine has the NewClassD discrete opamp stage), which can be set to 3/6/9 dB gain. I bypass the active stage to run the Hattor as a passive into the SMc buffer. However, I have tried it through the discrete opamp stage and I also find that to be an improvement over running it passively. Therefore, in ranking buffers (some with low gain) in my system, I would put the solid state SMc Audio buffer first and the Hattor Tube Active Stage second, both of which I would be happy to use. Third would be the discrete NewClassD opamp stage in the Hattor Big preamp, which I would judge as just a step below the first two. Finally, I just did not like the sound of the Tortuga or the Purity Audio buffers in my system.
Since you have an active preamp, you probably don't need a buffer. If you want to change the sound of your system, adding a buffer may be an inefficient way to do that. I suggest looking into a different preamp, or some type of EQ to deliver the type of sound you want to hear.
