Hello Marktdac
Laptops with SPDIF are relatively rare.
This may help you out with regard to which connection sounds best with your caimen.
http://www.headfonia.com/beresford-tc-7520-review/
I’m sure others here will have a different perspective.
This may go some way to answering your questions concerning bit rate and frequency. It’s worth book marking this site.
http://www.thewelltemperedcomputer.com/HW/USB_DAC.htm
Otherwise a simple net search should answer in detail.
I personally wouldn’t recommend using 3.5 jack to SPDIF converters and would recommend sticking to USB throughout.
There has been a lot of discussion concerning SPDIF v USB for audio in Hi Fi circles.
Generally most people believe USB is equal to if not better than SPDIF.
For laptop source applications USB has definite advantages.
Adding 8 bits to 16 bit sound files is unlikely to do any harm and may even give you a bit more headroom for noise. I use 24 bit replay for some 16 bit files in foobar2000 and ASIO4all. I can’t hear any difference.
Single spur balanced Mains. Self built music server with 3 seperate linear PSU, Intel i5, 16 GB RAM no hard drive (various Linux OS). Benchmark Dac2 HGC, single ended XLR interconnects/Belkin cable. Exposure 21RC Pre, Super 18 Power (recap & modified). Modded World Audio HD83 HP amp. Hand built Monitors with external crossovers , Volt 250 bass & ABR, Scanspeak 13M8621 Mid & Scanspeak D2905/9300 Hi. HD595 & Beyer 880 (600 ohm) cans.
The whole problem with the world is that fools and fanatics are always so certain of themselves, and wiser people so full of doubts.
-Bertrand Russel
John.