There are so many really good single box units today that it takes some major investments to upgrade the system beyond what they offer.
Also consider the fact that you then need 2 quality powercords and to get the most from a Dac and Transport a really high quality digital cable since the separates can only be as good as the cable between them - - major expense.
Excellent one box players on the market. Sim Eclipse $5200,
Audio Aero $6000, Mephisto 2 $7500, Accuphase DP 75 V $11,000, Alchemist product 8 24/192 $2400, and the best value in my opinion is the EMC 1 with 24/192 upgrade for $5000 a 50 lb beast that is as good as all the above mentioned players and perhaps a touch better. It does not have volume controls as the Accuphase or the Audio Aero but I do not consider volume controls to be a positive feature.
Hope this helps.