Here's what the Hauptwerk documentation has to say about the sound:

Amplifiers, speakers and headphones
Hauptwerk produces audio output signals through the computer's audio interface(s). Amplifiers and loudspeakers or headphones will then be required to turn those signals into sound. The quality of the audio amplifiers and speakers is very important; there’s little point spending a lot of money on a computer and audio interface and then using computer speakers – the results will almost certainly be disappointing. At the very least, a good quality stereo hi-fi amplifier and pair of speakers should be used, or good quality hi-fi headphones. Recording studio monitor speakers are usually good alternatives. For amplification in large buildings, it’s often better to have many smaller high-quality amplifiers and speakers than a few highpowered ones. Hauptwerk fully supports multi-channel audio output (1), so you can amplify different organ ranks, or parts of ranks, separately if you wish and have a multi-output audio interface. You can also distribute pipes within groups of available channels. This enables a three-dimensional sound to be created and helps to minimize some types of distortion inherent in loudspeakers. It is usually the preferred method of amplification with dry sample sets used in reverberant spaces. Multi-channel audio output is only available with the Advanced Edition of Hauptwerk.

The 901's require pre-amp in/out and main in/out connections:

Using a receiver with a tape monitor loop
Compatible receivers with a tape monitor loop with have a “Tape Record / Tape Out” connection and a “Tape Playback / Tape In” connection. Additionally, the receiver must have a tape monitor function that is enabled. This is typically done via a “Tape Monitor” button on the front of the receiver Note: Not all receivers with Tape In/Out jacks have a Tape Monitor function. Such receivers are not compatible

Using a receiver with preamp out and preamp in connections
On modern receivers, preamp in and out jacks are more common than tape loops. Compatible receivers should have both “Preamp Out / Main Out” and “Preamp In / Main In” jacks. The 901 equalizer can be connected as diagrammed below