Maybe Sacred Heart?

We had our wedding reception at the Depot's great hall.  It was really lovely, you could cater it yourself (although the kitchen is a smaller space) and we were able to bring our own booze.  To have alcohol, we had to hire a security guard and get a one-night insurance policy for the entire building but that wasn't too unreasonable.  We had 150 people at our wedding and there was plenty of space--you'd have to check on what the capacity is.  We had our wedding at the Rose Garden for a $25 rental fee for 2 hours but the kind Depot staff would have let us have the ceremony there if the weather had been bad.

I think 300 would not fit into the Depot.  I do know the people at 3Five Design (3FiveDesigns.com) do have a space for events and receptions at their building in Lincoln Park.  It looked pretty big, but I'm not sure about the kitchen and alcohol.  

There is also the Clyde Iron Complex that would definitely fit 300 and I know they were recently allowing free use of the facility for receptions in order to get the word out about the place.  However, I'm not sure if that offer will still be standing come February.  Might be could to check that out in the meantime.  Again, not sure on the alcohol/food thing, but I just wanted to throw out an option that would fit 300.

I'm in the process of wedding planning right now and I don't think we saw any venues that fit all your criteria.  We ultimately went with Holiday Inn since they had good room deals for guests, free parking for anyone coming to the event and discounted meals for kids.  They also had a complimentary room for me and my fiance, which didn't hurt.  The big ballroom for them also seats 300 so that was a plus.  

Good luck in your search!

It's a ways out but the Munger Town Hall might work for you.

Look to a park. You could do it in Lincoln Park for as little as $50 + a security officer (to have alcohol).

I think the Holiday Inn might be open to you having your own food catered and bringing your own alcohol.  I am not 100% on the policies, but the ballroom definitely fits 300.  You can call Kirsten at the sales office and find out.

Oops, I thought that said August 26th.

We had our wedding at the Duluth Aquarium in 2006. The price was reasonable, and they accomodate both outdoor or indoor ceremonies, making it ideal for unpredictable weather. They even feed the otters late so they are out to play when your guests are there. Exclusive catering contract with Bellissio's, and the food, service, and bar were great. The band set up under the giant carp/catfish thing in the entryway. It was unusual, but our guests still comment on it being one of the best weddings ever. If you will have a lot of guests with children, this is a great way to go.

Not the classiest joint, but the Superior Curling Club fits the size and catering needs. I'm not sure about their policies on bringing in alcohol.

Northland Country Club should also be considered. Its ballroom is not just for use by the members and Chef Mike does amazing things with food! Jenn Kolenda will help you plan every detail as well.