You need a phono pre amp for RIAA equilisation. Inexpensive models from the likes of ART, Rolls & B-Tech are a fine match for the gear you already have.
As to the problem of keeping your computer output and TTs permanently connected to the monitors there are a variety of different solutions each with their own pros and cons.
1. Get an integrated amp/receiver with a built in phono pre amp. This is the home hi-fi solution. Connect the TT to the Phono in. Connect the output from your DAC/PC to a line in. Connect the speakers via a line out, usually the tape monitor pair. This is the simplest and probably cheapest solution overall. The downside is that it is rare to find an amp with a line out that you can use to control volume pre speakers. Tape monitors are usually set at a fixed level.
2. Get a mixer. This is the Pro solution. You can still get DJ mixers withe built in phono pre amps. Otherwise connect the output from your own phono pre amp to a line in pair and the output from your DAC/PC to another line in pair. Connecting the KKRs is easy. This is how they were designed to be used. The advantages are easy volume control and connection. The downside is small form factor, good quality, clean and transparent mixers are not in expensive. Cheap ones can be noisy and not last long.
3. Get a multi channel audio interface. This is the clever,up to date solution. Connect the phono pre amp to a line in pair. Connect the monitors to an output pair. Use the direct monitoring switch and/or supplied software to switch between sources and control volume. A semi pro audio device will have the balanced connectors which are best for your KRKs. There really isn't a downside to this solution beyond buying the interface in the first place. Prices for decent kit start around $150. You can get genuine pro quality for less than $500.
4. Get a line combiner/distribution amp/DI box. This is the pragmatic DIY solution. Check out the big supplier websites such as http://www.thomann.de/gb/index.html or http://www.studiospares.com/page/home/ You can find a 4 into 2 line combiner from as little as $30.