Need a preamp?

Hi all, 

I have an older Cary tube preamp and a Bob Latino ST70 with a built in attenuator driving a pair of Proac D30R. The combo seems to be a tad too warm so I tried using the amp without the preamp and it seems to sound better with a wider soundstage and more extended highs. I have a phono amp also so I can also listen to LPs and cds. Is there a reason I need a preamp other than the convenience of switching between the two?  Would a ss preamp or passive preamp add to the sound?  I presume any preamp would only degrade the quality and can't make it better than it is. 

@atmosphere. That's very informative. Thank you. I'm glad there are science and answers behind the different perceived sounds. 
I got the 60 dollars simple Douk audio switch box and can't discern any differences. However, my system is not the most revealing. Not elegant but works. 
@jaferd  I'd argue that point- your system easily revealed one of the most common complaints leveled at passive controls. But the switch box doesn't seem to cause any harm?? That sounds to me like your system is revealing, and that in your case it is an elegant solution, even though it might not **look** elegant.