A321 Captain reported a fume event during preflight which necessitated a request for maintenance. Captain deplaned the entire flight crew and suggested a call to medical services if anyone experienced physiological symptoms.

Narrative

I was operating flight XXX; ZZZ [to] ZZZ1 on Date. The crew and I were waiting for the aircraft at the gate. Once the aircraft arrived and deplaned; a FA (Flight Attendant) came up and told us about how the APU was inoperative and effected their flight out of ZZZ2. She also stated how there was a smell in the aircraft; but didn't know if it was associated with a fume event or just a funky smell. The FO (First Officer) and I went down to the aircraft and put our luggage into place. Upon entering the cockpit; both the FO (First Officer) and I commented on the smell. The smell did not seem to at first be that of 'smelly socks' but more of what the FO and I described as a well used horse stall. Thinking it was just remnants of a previous crew; we went on with our checks. The FA's got on the aircraft and while preparing for the flight made a comment about the smell. Upon hearing this I walked the aircraft trying to smell for a different smell to see if it was a fume event or just a smell. After getting back up to the cockpit; I asked the FO if he felt any different. He said he felt a little off. At this time; I then told the entire crew to get off the airplane until we figured out the source. It wasn't until the crew was off of the airplane did I start to feel a little light headed. That was when I knew it was a fume event. I then got the logbook and the FO notified Operations that we had a fume event. We waited on MX (Maintenance) to show and I then was asking the crew if anyone felt sick. At first; the FA's didn't say anything about feeling sick. They decided to go back up to the gate and wait inside. I obtained the number for [contract medical service] and advised the lead FA that if anyone of them felt sick to call [the contract medical service] and to also advise their Supervisor. MX had shown up and was addressing the problem. During this; I called [the medical service] and after my phone call with them; they advise me to be checked out by medical professionals. Scheduling then called me because I had timed out and they released me from the flight. I left to seek medical care.