One meaning of "too closed" is that the reed, especially a lighter one will be touching the mouthpiece itself as it vibrates, coloring the tone and (in my experience) producing a fairly stuffy sound. In fact, it can be "close" enough that the entire opening closes completely as the reed covers the entire tip opening. It's a dance. I'm with some of the other folks here in that I play a Berg 115/3 medium with a #1.5 Plasticover reed (very soft.) For me, that allows me to utilize a lot more of the chamber, tip, and roll-off baffle to shape the tone. The reed is vibrating very freely. This takes more air to fill out the sound, but your lip might thank you for it when you are playing long or back-to-back gigs. The soft reed requires very little lip pressure to control it, but you do have to have a sensitive lip to make actual music out of it.