Dumb question......why do you need a preamp?

My MSB Discrete DAC has a volume control and plenty of output. Almost 4v at 300 Ohms and close to 2v at 150 Ohms. I use predicted 150 Ohm 2v setting and without a preamp. I owned a Benchmark HP4 preamp and I found that the sound was better without it. So I sold it. The MSB has a high quality volume control and I only have a digital source. 

For me the pre amp is the hub of all the components.

Streamer & cd player to DAC. DAC to preamp, tt to preamp, one pre out to sub, 2nd pre out to main amp.

I use a old recapped Carver c4000. Attenuators for high & low HZ, balance, filters, an awesume sonic holograghy feature which enhances soundstage.

Even has a built in amp for time delay speakers. I dont use it anymore but it gave a nice concert hall effect or " opened up" a small room.

Added gain, drive, dynamics, energy and sound stage. 

I have a luxman L-507z integrated amp. I havent seen anyone put a tube pre-amp on it yet. Altho with cornwalls it might be interesting. 

@paqua123 - The Luxman integrateds really are something special.  Many manufacturers claim that their integrateds have their top of the line preamp circuits built in and who knows but in the case of the Luxmans I honestly believe it.   The difficult thing for me is that I'd actually like to go fully active, and trade from the Luxman integrated to Luxman pre, but the pre's are like 2x as expensive as the integrateds.

Having enough output voltage from a source component to drive the amplifier(s) is a good start, but may not be enough for some listeners depending on their equipment, set-up configuration, and listening preferences. Listeners who use passive volume controls, either inside or outside of the DAC, typically value the clarity and absence of noise they perceive from removing the active stage from their signal path. However, even with source components having enough voltage to drive the amplifiers, impedance differences between source and amplifier, and cable length, can affect the sound. Improved dynamics, body, and tonal qualities are what I read about most from folks who switch from using passive volume controls to an active stage, or a transformer/autoformer based solution.

Of course, active circuitry and transformers add their own sonic changes (and potentially noise) to the signal so, the trade-offs are sometimes described as improved clarity and lower noise when using a passive solution vs. improved body and energy when using active circuitry.  Choices include either:

• Live with the impedance relationship between the source component (usually a DAC) and the amplifier(s) and minimize the effects of impedance by keeping IC cables short and selecting equipment with low output impedance and high input impedance, or
• Utilize an active stage (or transformers/autoformers) between the source and amplifier(s) in order to improve the output/input impedance relationship.

In cases where the source output voltage is sufficient, the active stage can be as simple as a unity-gain buffer (you would not need more gain), or transformers/autoformers, which can be set up for unity or other levels of gain up to about 6dB. Examples include totaldac’s d1-driver-sublime, which I believe is basically a low (or unity) gain active stage, or Empirical Audio’s Final Drive, which are a pair of passive transformer buffers/selectors. In both examples, the manufacturer’s goal was to insert a device in-line that optimizes impedance matching in order to improve the sound of their DAC when directly driving amplifier(s).