I'll suggest adding a good preamp at least once to all setups. If, after trying this, you don't notice any improvement, sticking with the direct connection might be the best choice. However, like myself, if you experience a noticeable improvement, it's unlikely you'll go back.
After integrating Parasound Zpre2 and Zpre3 into my systems, I shifted from the 'No preamp is the best preamp' belief to becoming a preamp believer. What's remarkable is that these enhancements didn't break the bank.
The top ends exhibit more airiness, the mids are fuller, mid-bass has greater extension, and the low-end notes carry more weight. Even the control over pops and plosives is notably better. These sonic improvements transcend simple coloration. Audiophiles often attribute them to 'synergy,' although the specific elements contributing to that synergy remain a mystery to me. I hope that someday, a knowledgeable audiophile might shed some light on this.
While it's possible to manage without a separate preamp if your DAC or streamer has a good preamp section, most DACs/streamers lack this feature.