PSB B-15s
Yes, the PSB's are excellent speakers. I have a pair in my current two-channel setup. I have the B-15s, but the B-25s are probably a better option.
What electronics are you using to run the setup? While the use of a sub is good, if you have the space for floor-standers, you should look at the Image T-45 ($750/pr) or T-55 (950/pr). They will hold their own in the Bass department and save you money on stands for the monitors.
For a monitor setup, you are looking at:
Monitors 400
Stands 100
Subwoofer 400
The monitors will require that you find a good place for the subwoofer to go, which is not always feasible in some rooms. Properly blending a sub and satelite combo is relatively difficult. Further, your mom may not like the presence of a subwoofer for aesthetic purposes. Decent towers make for a simpler and more economical setup. Unless they like organ music, 33Hz is as low as you need to go and the T-55 will get you there.
I DO NOT recommend mounting the speakers on the walls, even though the manufacturer says it is possbile. The best sound and imaging comes from speakers at least 18" away from the walls. There are many posts on this subject.
If you haven't decided what to drive them with, look at the Outlaw RR2150. 100WPC, bass management, subwoofer out, and extreme flexibility (USB, mini pin in) make this an excellent option. Since this is for your parents, they may also find the styling has an attractive nostalgic quality.
For a source,
Low Budget: Pioneer DV588-A ($125) local store
Med Budget: Onix XCD-99 ($300) av123.com
High Budget: NAD 542 ($500) and you get to be pure canadian!