Subwoofer for classical music listener

stfoth:  Yes, except for lacking deep bass, I was happy with the system before I added a sub (but, for other reasons, was not happy when I hooked up the sub the normal way.)
With the high-pass I started experimenting, alternately physically removing the sub and putting it back into the system.  During this process I sometimes noticed exaggerated highs even without the sub.  That's why I'm suspecting a bad tube may be developing in the preamp.  Alternately, sometimes the system sounds fine with the sub connected.
 I recently had the amplifiers upgraded, so I don't think there's a malfunction there.
It's not an exact science.
 I'm trying to cover all bases.

I apologize in advance.  You are all audiophiles and I'm only a beginner - and a poor one at that.  But I am desperate for some help.  Finding the right speakers and receiver, on a budget, for CLASSICAL music and OPERA is very, very hard.  I have a 13'x13' family room and $600.  I'd like to plug my computer, dvd player, tv, cd player and speakers into the receiver.  Please don't hate me for getting on here to ask for help.  You men/women are music gods and I need some miracle.  I'm obviously poor and don't know a thing. Right now I have Infinity Reference 2000-3 speakers and a Yamaha RX-395 receiver - if I plug my MacBook Air into that it really sounds bad.  I've been listening/watching opera via streaming/dvd on my computer with headphones.  A 3-hour opera with headphones is long.  Anyone care to help?

@shaunac,
You would do better to post a new thread.
B

Sennheiser HD600 for the beginner