Before to use MIDI it is preferable disable appletalk and system extensions that make use of the modem or printer port.
The new Internal MIDI manager and the full OMS (1.2 or better) support permits to receive/transmit MIDI sample packets from/to most samplers using most MIDI interfaces, also with multiple out/in routings.
In all cases avoid midi thru or midi feedbacks around your network.
I fixed a bug in version 2.3 which was causing an counter overflow in a midi message. Now the MIDI handshaking works fine and it is fast also for very long files (also using the internal MIDI manager).
If it is possible, please send to me an E-Mail describing your configuration (mac, interface, synths/samplers) and if it works properly.
I tested D-SoundPRO 2.4 on SY99, TG500, AKAI S2000 using a PowerMac 6100/66, Studio 4 MIDI interface & OMS 2.1.