There are several so bear with me if I miss some.
Agile Auto-Bel Air They are Cobb ProTuners and do install/builds. They also do full Subaru maintance. Very friendly staff (plug for Seeing Stars) and overall have very little complaints for the most part. They will/can explain different setups/options.
IAG-Westminster A new shop but they seem to stock a LARGE amount of Subaru parts. I havent been in there yet but it is on my agenda. They seem very friendly as well and seem open to exploring and chatting about setups. They will probaly have an Open House in the spring.
Andrewtech-Gaithersburg If you need any tranny work take it there. They also do basic maintance and trouble shooting. Grade A shop
TurboXS-Gaithersburg Good people to go to for Utec tuning. I do not have ANY experiance or a ton of second hand exxperiance with this shop.
Annapolis Subaru-Annapolis Andy who has since stopped working there was a top notch salesman. Jackie who works in the parts department is very helpful in finding hard to find parts. One of the best local dealerships I've delt with.
Fredrick Subaru-Fredrick They are active on Nasioc forum a fair bit. I'll probaly pick up my GF's next car from them due to this. Kudos for supporting the community.
Race Comp Engineering-Columbia Suspension/brake gurus. Myles has a ton of experiance with racing/suspension. They also recently hired a former Subaru tech with a ton of Certs. They also sell parts.
If you have any questions feel free to ask. If you are a Mid-A vendor in the Baltimore area and I missed you I am sorry. It's 11:29, I've been up since 4:30...and I'm tired.