Do I need a special DAC for SACD ?

With a few exceptions the DSD signal can't be routed through a digital output to an external DAC. You need a full SACD player that connects through it's analog outputs and can also be used for CDs. The CD digital signal can be routed digitally to an external DAC if desired.

I cannot imagine any reasonably priced SACD player in any way approaching what you are hearing out of redbook CDs on the MBL combo you have. If you just want to fool around with SACDs to see what it's all about, I suggest a used Sony XA-9000ES SACD player. The redbook isn't all that good, but at least it will give you somewhat of a reasonable rendition of SACD sound. I also see an XA 777 up for sale, which is another good SACD player. Given that you are used to the sound of your MBL, I think going any cheaper on an SACD player would just be a waste of time and expense for you.

K, are you wanting multichannel or simply higher resolution? Multichannel is probably the most noticeable benefit you will gain over your MBL setup - and there simply aren't that many titles to make it worth the expense.

If you want to dabble, I agree with Chayro, try the Sony (used if possible) which will give a decent understanding of the differences. In a pinch, pick up an Oppo for under $300.

While none of the hi rez formats will sound up to par with your MBL on these machines, you can hear the difference at lower fidelity and assume some differences upward. However, the difference between hi rez and red book will be much smaller once you start getting into MBL range equipment.

By and large, high res formats best serve music with lots of instruments in similar frequencies and also help some systems better render high frequency sounds like cymbals or high pitch woodwinds. Classical is probably the most popular genre for hi rez for this reason.

I own several SACDs of popular titles (Dire Straits, Police, etc) and notice minimal difference on my system. It certainly doesn't affect the vibe of the music the way listening to CD versus vinyl does. Nor does it offer the resolution difference between 33 rpm vinyl and 45 rpm vinyl.