What can I improve in my audio system for better s

I have lossless FLAC music ripped from cd's and music I purchased from HD tracks, I am using Macintosh MX 121 processor, Krell S1500 7 channel power amp, 7.1 SYSTEM, B&W 802D speakers for front and center, Monitor Audio for side and rare channels, Crystal and Audio Quest for interconnect and Tara Lab for Speaker cables. I am using JRiver to play this music from my computer, which resides on Lenovo Q180 small foot print PC with build in HDMI which is connected to my processor via this HDMI. I don't know why but I feel that this PC is the least expensive equipment I have, and is the weakest link in my entire system. There is nothing wrong with the system. But, it always bugs me that I can improve this weak link. I like the fact that while playing music I can see the album cover and track information on the monitor. But still I am curious to know, if possible, how I can improve the sound quality. Since I have a McIntosh processor with built in DAC, I don't want to add another DAC (by the way I tried using Moon DAC, it was no-ware close to McIntosh MX121, ended up selling it on eBay). I am not even sure, while using HDMI port does better sound card matters? Or can I enhance anything in my system to improve the sound quality or am I looking for unrealistic Magic? When I purchased this system, my friends were so surprised from the money I spent, there first question to me was "Does singer comes out of these speakers to sing for you?" I guess that's what I am expecting; I believe I am missing those emotions that we get from live performance.

Need a professional advice, please help.