you already have a soundblaster zx. while its not as good of a dac as the dx has, its mid level so not terrible. a dac like a schiit modi is certainly an upgrade for 2.0 audio but the question of "if your ears can hear it" must be raised. some people can not notice it, while others think its night and day. its the "how much of an audiophile are you" million dollar question.
wharfdales are passive.. instead of a dac for them you would want a receiver (basically thats a dac+amp in one box). you could just use an amplifier, but the cost of decent power amplifiers is high so a stereo receiver is often the cheaper choice.
you can use a dac with any powered speakers such as i listed above. or, anything on this list id say
http/www.musicradar.com/news/tech/21-of-the-best-affordable-monitor-speakers-570959 though i'm not familiar with them all.