D18 is getting old by now, not sure they still make them any longer. Still a great sound if you want something darker, warmer, more full bodied, but it is lacking some modern options if that matters to you (like USB and DSD capabilities for example).
If you want smooth for a more current DAC, you could go for the Cayin iDAC-6 which has a tube buffer and adjustable digital filters. That DAC can sound smooth and warm or neutral if you choose solid-state mode and certain digital filters. Pricing is not too far off from what the D18 used to go for when new, and I've seen used examples go for great prices.
You also might check on the Audio GD models like the latest DAC 19 or especially the NOS 19.