Best DAC

@mr_sacrimoni, Yes, depending on your other sources (if any) and the DAC, you might not need or want a preamp. Often times with DAC’s the volume control whether on the DAC or preamp acts more like a brake than an accelerator. Many modern DAC’s have an output voltage of about 2.1 Volts. Your Threshold S500 has an input sensitivity for full output at a hair above 2.1 Volts. Many modern DAC’s have their own volume controls. Many modern DAC’s have 32 bits available. Most hi Rez digital media is capped at 24 bits. These high bit DAC’s attenuate at about 12 dB per bit. So there are plenty of extra bits available to control volume with little concern for bit stripping. Running such a DAC straight into your Threshold S500 will avoid the unnecessary additional gain stage, additional volume control, and interconnects of and for the preamp allowing for a much purer signal path.
The sale of your preamp might go towards funding your new DAC.

A few thoughts: if you have the space; you might consider using  some of the funds towards bigger maggies. Also, when was the Threshold S500 last recapped? Depending on the DAC you choose; you might find the SP6 superfluous.