Is a low load resistor a big problem in low pass filter and amplifier circuit
Your speaker rated at 1W will require an amplifier circuit to be placed between the DAC / filter circuit and the speaker. There are numerous solutions to provide for this amplifier function including discrete circuits, IC circuits and single chip amplifier modules. You could build up a circuit using a chip amplifier such as an LM386 or you could check online sources to get a low cost ready-made circuit module. Sellers on eBay are making an assortment of modules available such as this example: