Good speakers for a low budget

Usher or PSB are good thoughts I could support as well!

Magnepan mmg or Triangle Titus.

I prefer to know more before recommending usually, but there are certain models in this price range that a layperson can surely build a nice system around and I have no problem pointing someone to these and then letting them decide. It beats the alternative of picking randomly based on whim because there is also a lot of junk out there for this price.

As much as I like Apogees, I would caution that they are usually hard to drive correctly and an atypical design/form.

Knowing more on the application would definitely be important in the case of recommending Apogees.

I'm also thinking buying another pair of speaks for $600 will not make much difference from the current PSBs, if this model PSB sounds like others I have heard.

If the CD has a digital output, an outboard DAC might be the best investment for $600. An mhdt Paradisea can be had for under %500 and make a real difference in the sound.

If she's using a stock RCA interconnect between CD and receiver, maybe upgrade that to a decent Audioquest or similar IC for under $100 also.