In my opinion, the pre-amp first. I would get the best pre-amp I could afford and build from there.
The Amp also depends on speakers selected. So there is a back and forth involved.
Hi efficient speakers don't need arc welder style amplification. Or, not so efficient speakers need more powerful amplification or if the speaker's impedance is low, then you need specific amplification to drive them correctly.
But, the pre-amp is typically not system dependent.
Also, yes, there are digital sources that have volume controls incorporated. However, they typically are not internal pre-amps and therefore are not control amps that have multiple inputs. So if you decide to have more than one input source, that digital source with a volume control really doesn't cut it.
You have an SP16 pre-amp. That pre-amp has a decent incorporated phono section. So, you currently have multiple inputs, volume control and a decent phono section.
My recommendation is to "borrow" a REF 3, REF 5SE, REF 6. Take them home, and listen in your system doing an A/B comparison with your SP16.
I did that with my SP9 MKII, SP11, REF 3, REF 5 and REF 6. If it wasn't for wanting remote control capability, the SP11 is outstanding.
However, the REF 3 is very nice indeed, even compared to new pre-amps. You can't go wrong with a REF 3. The REF 5 vs the REF 3 didn't show enough of a difference to warrant (justify?) upgrading to the REF 5. Just not that much of a sonic improvement.
however, the REF 6 vs the REF 3 was a noticeable difference. Then I got a deal for a REF 10 (the best) that I could not pass up.
looking at your current system, The only thing i would consider upgrading at this time would be the pre-amp. Go for a REF 3 or a REF 6.